Blogging ICHEP 2010

End of part two

2010-08-31T21:19:00.003+02:00

All good things come to an end (some bad ones too) . Among the many duties of the organisers of a conference, the last one is to close the conference "officially", as well as all the peripheral activities... like this blog.

Since all conferences are as much a human adventure as a scientific one, there are a couple of thanks that I would like to address before switching the lights off.

First of all, thanks to the LHC, and all the 4 experiments, for having worked so hard to provide first results at the ICHEP conference. We hoped for these data since we thought of organising the conference in Paris, and we followed the ups and downs of the machine with mixed feelings. Would there be anything to show ? It turned out that all the collaborations worked very hard on the few months of data-taking, and obtained quite remarkable results (I am still impressed by the mu-mu spectra of ATLAS and CMS, and their nice resonance peaks).

Thanks also to the Tevatron, CDF and D0 for providing us with data that keep us wondering if something really new is just around the corner, trying to understand better the Bs-meson, pushing the limits on the Higgs boson... The race is not finished, and we should have rather interesting discussions across the pond very soon.

Thanks to all the technicians, secretaries, researchers (staff, post-docs, students) who helped in organising and running the conference. You cannot have a clear idea of how a conference of a thousand people will run... until it starts, with your fingers crossed, hoping for the best. Thanks to all of them, things ran very smoothly at each step (during the juggling of the parallel sessions, the ceremonial of the plenary talks, and even, yes, for the visit of N. Sarkozy).

Thanks to all my colleagues of this blog, who succeeded in giving a comprehensive (and sometimes very detailed) impression of ICHEP. I am quite convinced that people following the conference through the webcast and the website got a much more lively and interesting view of ICHEP thanks to this blog. Probably a lesson to remember for the next editions !

Thanks to the participants and speakers (which is approximately the same crew) for coming and taking part in this conference. Even though there were not many questions (in particular during the plenary talks), I caught many people at coffee breaks -- or during sessions -- in intense discussions not only about the food, the president, or the ICHEP bag, but also physics...

And finally, thanks to you, the readers of this blog, for your comments and suggestions all along this adventure. We hope that you enjoyed your time here. We definitely had a wonderful time sharing our two pence of knowledge with you...

"See" you in two years !

The calm after the storm

2010-08-09T15:37:00.000+02:00

Marco, Barbara, Georg and Jester's summaries didn't leave much room for improvement. But here's my take - in the weeks and months following ICHEP, I'm sure we'll hear much more from the LHC. The collider's performance continues to improve (this weekend's milestone was the first inverse nanobarn delivered to the ATLAS and CMS experiments). Fermilab will continue to make the most of its last year(s?) of Tevatron data. New neutrino experiments will come online, existing ones will deliver some much-anticipated data, ditto for ground-based and space-based particle astrophysics experiments. And we'll all look forward to the next ICHEP, in 2012 in Melbourne, where we'll hear the latest in theoretical and experimental particle physics, see what the future looks like two years later, and....get a new ICHEP bag. As I've discovered, not only do they make excellent laptop carriers, but they also work pretty well as beach bags.

Very Last Summary

2010-08-08T17:48:00.004+02:00

This is my last entry on this forum: my summary of the conference...better late than sorry :-)

Most Important Result: The Higgs exclusion limits from the Tevatron, of course. Anytime now we may get the answer to one of the most important question in particles physics. Not this time yet, but the thrill is on.
Most Intriguing Result: the forward-backward asymmetry of top decays at CDF has been updated to $15 \pm 5$ percent and lingers 2 sigma away from the SM prediction of approximately 5 percent.
Most Relieving Result: the poster from the HARP collaboration saying that the LSND anomaly was due to underestimated contamination of the beam with anti-electron neutrinos. If confirmed, that would solve the 10-years-long puzzle what went wrong in LSND.
Best Presentation: Nicolas Sarkozy. Gee, this guy knows how to talk, especially when contrasted with mumblings physicists. What fervor, what mimics, what gestures (ok, forget the jokes).
Best Presentation, seriously: Ben Kilminster, Higgs limits from the Tevatron. Maybe it's because when holding the remote he looks just like Colin Farrel in Bruges, or maybe because the presentation was clear, concise, and illuminating.
Worst Presentation: summary of BSM searches. Unfortunately, good experimental talks are rare. The cardinal sins are too much material, overcrowded slides, superficialness, no attempt at explaining presented results or methodology, and misleading theoretical interpretation.
Best Animation: the Planck satellite sweeping the sky while uncovering the temperature map. That was just lovely.
Best Music: given the number of cell phones in the audience, the competition is always fierce in this category. But if what I heard on the first day was really Genesis' Firth of Fifth, that obviously trumps anything.
Overall Impression: Even though and Paris is always worth a mass, the conference was pretty well organized, and I had fun at times, my opinion about the ICHEP series has not changed. Conferences with 1000+ participants are dinosaurs; more a brontosaurus rather than a T.Rex. Parallel sessions contain some interesting material, but the shortness of the talks and no time for discussions preclude any deeper insight. Plenary sessions on the other hand are typically hasty and overloaded summaries of what we already heard in the parallels. Alas, one needs an astereoid strike for dinosaurs to be replaced by more flexible mammals....so maybe see you again in 2 years, upside down ;-)

My Bet ? A Fourth Generation!

2010-08-04T22:28:00.002+02:00

What picture should we draw of the quest for new phenomena after the presentation of a wealth of new results at the international conference on high-energy physics in Paris held last week ? I am speaking in particular of results coming from the experiments at the Tevatron and LHC, which are all studying hadron collisions in search for still unseen effects to both confirm (with the discovery of the Higgs boson) or break down (with the observation of Supersymmetry, new particles, extra dimensions, or still other effects) the present theoretical understanding of fundamental physics which the standard model provides us with.

In short my question today is, on which signal or phenomenon should we place our chips if we were to bet that the standard model is finally going to break down ?

I have my own answer. But first, before I give it to you, I feel compelled to be extra careful in a couple of ways.

The first way is dictated by personal reasons: I want to state it here very clearly, because I often get fingered as a rumour-monger or overhyper these days. I do NOT believe that the standard model is breaking down any time soon. I have a feeling that we will have to live with it for a while longer. I do not believe in Supersymmetry at arm's reach or anywhere else, nor in other exotics signals that we might see with present-day machines.

(And, since I am going to talk about something like that in particular below: I do not believe we are going to discover a fourth generation of fermions any time soon; I believe the present 2-sigmaish excesses of CDF and DZERO searches for a new t' quark are not due to a signal. If you really want my opinion... they are due to a coherent underestimation of QCD backgrounds, whose root is the use of the same methodologies by the two experiments!)

The second statement consists in my disclaimer, which I will state today as follows:

"The opinions expressed in this article are those of the author, and they do not reflect in any way those of the institutions to which he is affiliated. These include the CDF and CMS collaborations, as well as the Italian Institute of Nuclear Physics."

The above disclaimer is directed in particular at science reporters and other information recyclers... Which should not mistake me for an official source of the experiments in which I work! Of course it is a insufficient shield, but at least nobody can say I have not been clear on the matter.

Okay, now I feel more free to discuss in enthusiastic terms what I think is the single most exciting and promising deviation from standard model predictions that we have in our hands at present: a tentative signals for a fourth generation quark!

Can Fourth-Generation Quarks Really Exist ?

I have kept my eyes open on searches for a new quark since 2008, when a CDF analysis showed some intriguing high-mass events and a vague deviation of data from backgrounds. (The post linked above is rather well written if you need some introduction to the physics!)

After CDF performed the same analysis with doubled statistics, again finding an excess of high-mass events, I thought things were really interesting and I said so here.

In the meantime, there was an enlightening paper which came out in the Cornell Arxiv. Titled "Four Statements About The Fourth Generation", and signed by distinguished theorists, it explained clearly that contrarily to what one might think (or read in the Review of Particle Properties, which makes several assumptions in order to state that a fourth generation is excluded by electroweak measurements), a fourth generation of fermions is not ruled out by experimental measurements, and might actually be useful to explain the amount of CP violation we observe in particle decays. I summarized the paper's highlights in another post which I think is worthwhile reading, if you are interested in the topic.

Well, now DZERO has published the results of a quite similar analysis, and it looks like they too see some excess in the same kinematical distributions that CDF used to search for a fourth-generation quark. Again, this effect can be easily understood in terms of background fluctuations or a mismodeling of the high-mass tail of some of the contributing processes. Yet, the coincidence of the two search results warrants some additional thoughts. So let me first of all show what DZERO has just made public.

The DZERO Search For Fourth-Generation Quarks

DZERO has published, in time for ICHEP 2010, a new search for up-type fourth-generation quarks decaying to W bosons and down-type quarks. In a nutshell, the search considers events of the "lepton plus jets" type: the same kind of events on which all the most precise measurements of top quark physics at the Tevatron are based.

In the lepton-plus-jet topology, top quarks are produced in pairs, decay to a W and a b-quark, and then one W yields two hadronic jets, while the other decays to an electron-neutrino or muon-neutrino pair. This results in one neutrino in the final state, which adds some complexity to the reconstruction of the kinematics (the neutrino is undetected, and only its momentum components transverse to the beam direction can be inferred); however the advantage of having one high-momentum lepton in the event instead of purely hadronic jets is a more than adequate payoff. The events thus must feature a lepton, significant missing energy, and four hadronic jets: backgrounds then are small; the largest is the production of a W boson plus hadronic jets.

When searching for a fourth-generation quark, DZERO does exactly the same thing as in top searches: they assume that the t' quark is produced in pairs, and that it decays 100% of the time into a W boson and a quark (not necessarily a b-quark). The final state is the same as that of top searches, save for the fact that the larger mass of the t' grants a slightly tighter cut on the energy of the leading jet, a device which further reduces backgrounds.

In the end, the data allow the reconstruction of a tentative t' mass, assuming that each event is of the t'-pair-production kind. A kinematic fit searches for the combination of jet assignments to the decay partons which best matches the hypothesized process. One thus obtains a histogram of reconstructed t' mass:

In the figure, you can see with different colours how the predicted amount of events coming from different processes (top pair production in red, W+jets production in green, and multi-jet production in grey) distribute in the reconstructed t' mass. The data is shown by black points with error bars, and it matches very well the predicted shape of backgrounds. An example of what contribution would be given by a 300-GeV t' quark in the histogram is shown in yellow. Tiny, but not entirely undetectable. Mind you: the vertical axis has a logarithmic scale!

What is maybe not so immediate to discern from the figure is the fact that while backgrounds have a wide distribution in the reconstructed t' mass, the signal of a t' quark if present would populate a narrower region: the one around the real mass of the quark. This is entirely the point of having constructed this kinematic variable -discriminating signal and background.

A second discriminating variable is the sum of all transverse energies of the observed final state objects: jets, lepton, and missing energy. This is the so-called "Ht". Ht is large for processes that involve the production of massive states, and so it is a good means to separate t' production from the top and W+jets background. Below you can see how the data compares to backgrounds as a function of Ht; the color coding is the same as above.

DZERO performs a fit in the two-dimensional plane of the t' mass and Ht to extract the possible amount of signal present in the data. This is performed as a function of the unknown value of t' mass: since the distributions of reconstructed mass and Ht of the signal depend on the true t' mass, several fits are performed in series, to extract a limit curve which depends on that parameter; the curve is investigated by points, at 25-GeV intervals in the unknown t' mass.

The result of the fits is displayed in the figure below. The t' mass (this time the "true" one, not the reconstructed tentative mass of the kinematic fits) is on the horizontal axis, and on the vertical axis is the production rate of the fourth-generation quark pair. The black line shows the theoretical prediction for the rate, which falls quickly as the t' mass increases: fewer events are expected in the 4.3 inverse femtobarn dataset of analyzed collisions as the t' mass increases, because the higher the mass, the more energy is required to produce the heavy quark.

The theoretical curve of the signal cross section can be compared with the red curve, which shows the upper limit (at 95% confidence level) extracted from the data. The red curve lies below the black one for low masses: a light t' quark (of masses below 296 GeV) is excluded by the data, because it would have been copiously produced in the Tevatron collisions, and would have stuck out in the two tested distributions. For higher mass values, the limit is above the curve: these mass values are still possible.

Now observe the blue and yellow band: these describe what rates of the searched quark DZERO expected to limit, as a function of t' mass, given the amount of analyzed data they had and the analysis strategy. The blue band shows 1-sigma variations in the expected limit, and the yellow band shows the range of 2-sigma variations. In practice, the bands pictorially explain what "on average" would result from the search, if no signal were present in the data.

Now, the red curve stays on the edge of the 2-sigma band for masses above 300 GeV. What this means is that DZERO has a slight excess of events which distribute like t' production ones in their data. Not awfully exciting, I'll admit. But now compare the curve to the one found by CDF just a few months ago (the analysis which I have discussed in detail here, as already mentioned):

CDF found a strikingly similar result! True, CDF had more sensitivity, so their limit is slightly better; but the behavior of CDF data and DZERO data is indeed quite similar. A fortuitous coincidence between two 2-sigma results ? That is surely a possibility; another one is that the two experiments, which rely on similar simulation tools, both underestimated the high-energy production of top or W+jets production events.

Yet a third possibility remains on the table: that both CDF and DZERO are seeing the first hint of pair production of a fourth-generation quark. The amount of data of the two experiments would be insufficient to see a clear signal yet, so the first hint is just that they both obtain a mass limit well below their expectations.

Now, suspend temporarily your disbelief and consider. If a 400-GeV t' quark exists, who is going to discover it first ? For sure CDF and DZERO with twice as much statistics (which they almost already have in their bags) would be likely to make those 2-sigma excesses become close to 3-sigma ones. Maybe adding other search channels would further increase their reach; but they would probably be unable to conclusively discover the quark.

Instead, on the other side of the Atlantic Ocean... CMS and ATLAS would be very fast in finding conclusive evidence for such a quark! The reason is that producing a 400-GeV t' quark at LHC is much, much easier, given the over 3.5 times higher energy of the LHC collisions. The cross section at the LHC is of several picobarns, which means that well before collecting an inverse femtobarn of collisions, the CERN experiments will find the new quark!

Now, let me say something personal, deep down this long post. I have always said that, despite I have been working more on the CMS experiment at CERN than on the CDF experiment at the Tevatron since 2008, my heart still beats stronger on the Tevatron side... That is still true in a sense: CDF is such a fantastic achievement for science that I will always be proud of having contributed to it for 18 years (and counting). But if you ask me which experiment I would prefer to see discovering a t' quark... I would say CMS!

The reason ? CMS and ATLAS deserve to become the focus of the next decade of high-energy physics research. Too much has been invested in human resources for these experiments to fall short of being a total success. I would love it if the adventure of the LHC experiments into the unknown were to start with a t' discovery, early next year! It would be just great!

... But now please go back and read my original disclaimer once more!

Things you see and things you don't see...

2010-08-03T09:49:00.003+02:00

It's already more than a week ago that I saw my first president at a physics conference. Doesn't time fly? There were so many things to see (or not) during ICHEP that it really stands out from other conferences I have been to. Hey, after all it was the first real big one with first real LHC results, after Physics at the LHC at DESY, which didn't have quite as many participants. Last year, at the Lepton Photon conference, the main conclusion after every talk was: "We are looking forward to results from the LHC!" It's great to see that those times are over and that the community is buzzing over limits and cuts and simulations and candidates!
Of course what we didn't see was the Higgs. Many people thought we would (which meant we also saw more journalists than ever before at a physics conference), and now the next big question is: what's next? Will the Tevatron keep running for another three to four years? Will that mean it will see the Higgs? From what I hear, that's not a given, but it'll certainly be an exciting time.
Some people also saw the film Sunshine during the nuit des particules at the Grand Rex, and at the time time saw a lot of the actress Irene Jacob - that dress, and a story about balls of fire in a kitchen will go down in particle physics history.
Now it's time to see what's next - for me, that's the global Particle Physics Photo Walk next Saturday. More than 200 amateur photographers from around the world will get an exclusive look behind the scenes of five physics labs (KEK, CERN DESY, Fermilab, TRIUMF) and we are very much looking forward to see our labs through their eyes.

Summary of personal impressions

2010-08-02T12:38:00.004+02:00

In looking back at ICHEP, what are my personal overall impressions? The conference was very well organised (with the minor exception of the dinner), the venue was great (as were the conference bag and its contents), the President felt obliged to attend -- so it was clearly a good and successful conference overall.

It was also pretty big (for a physics conference, not in the greater scheme of such things), in fact almost too big for my taste; perhaps it's just that as a theorist I'm naturally more introverted, but I find it difficult to meet people and start a conversation when there's a huge crowd. Smaller more focussed conferences are probably better for discussions; there was also a notable lack of questions in the plenary talks -- perhaps also a symptom of excessive size.

On the other hand, the huge size means a very diverse set of speakers, which enables one to learn about all the things that have recently gone on in the wider field. Since the arXiv is getting so vast that it is well-nigh impossible to even read the titles of all papers that get posted to the hep-* sections (much less the abstracts, to say nothing of the papers -- even assuming that one had the exceptionally broad knowledge base to be able to make sense of all of them), this overview is perhaps the most important function of a large conference like ICHEP.

And the things to be learnt were of great interest: CMS and ATLAS have "rediscovered" the Standard Model; that in itself is no surprise, but the speed at which the LHC experiments have managed to get there is amazing at least for this theorist. The arrival of the LHC hasn't rung the death-knell for the Tevatron quite yet, though: while rumours of a Higgs discovery turned out to have no foundation in fact (a 2σ deviation is hardly a basis even for a rumour), CDF and D0 combined could exclude a much larger mass region for the Higgs, further narrowing down the regions where it can hide. Also from the Tevatron comes the like-sign dimuon charge asymmetry that may be the first sign of new physics if it is confirmed by another experiment. Away from the big colliders, the neutrino physicists and cosmologists are also doing impressive work and chipping away at the Standard Model's plinth. The representation of my own field of research was perhaps not optimally suited to the audience, since the parallel sessions on lattice QCD were not very well-attended except by the lattice people and the plenary talk concentrated on work that would likely have enraptured a nuclear physics audience, but probably not a HEP one. Overall, I got the impression that the experimentalists take ICHEP much more serious as a forum than we theorists do -- there were a lot of new experimental results presented for the first time at ICHEP, whereas most of the theoretical results had been presented at other conferences or been posted on the arXiv earlier.

Blogging a conference as part of a group rather than on my own blog was an interesting new experience; for a lone blogger, ICHEP would have been way too big!

Meanwhile in the South: CoGeNT dark matter excluded

2010-07-31T15:50:00.007+02:00

Parisians say that il n'y a que Paris. This is roughly true, however ICHEP'10 in Paris was not the only important conference in France last week. At the same time down south in Montpellier there was the IDM conference where a number of results in dark matter searches was presented. One especially interesting result concerns hunting for light dark matter particles.

Some time ago the CoGeNT experiment noted that the events observed in their detector are consistent with scattering of dark matter particles of mass 5-10 GeV. Although CoGeNT could not exclude that they are background, the dark matter interpretation was tantalizing because the same dark matter particle could also fit (with a bit of stretching) the DAMA modulation signal and the oxygen band excess from CRESST.

The possibility that dark matter particles could be so light caught experimenters with their trousers down. Most current experiments are designed to achieve the best sensitivity in the 100 GeV - 1 TeV ballpark, because of prejudices (weak scale supersymmetry) and some theoretical arguments (the WIMP miracle). In the low mass region the sensitivity of current techniques rapidly decreases, event though certain theoretical frameworks (e.g asymmetric dark matter) predict dark matter sitting at a few GeV. For example, experiments with xenon targets detect scintillation (S1) and ionization (S2) signals generated by particles scattering in a detector. Measuring both S1 and S2 ensure very good background rejection, however the scintillation signal is the main showstopper to lowering the detection threshold. Light dark matter particles can give only a tiny push to much heavier xenon atoms, and the experiment is able to collect only a few resulting scintillation photons, if any. Besides, the precise number of photons produced at low recoils (described by the notorious Leff parameter) is poorly known, and the subject is currently fiercely debated with knives, guns, and replies-to-comments-on-rebuttals.

It turns out that this debate may soon be obsolete. Peter Sorensen in his talk at IDM argues that xenon experiments can be far more sensitive to light dark matter than previously thought. The idea is to drop the S1 discrimination, and use only the ionization signal. This allows one to lower the detection threshold down to ~1 keVr (it's a few times higher with S1) and gain sensitivity to light dark matter. Of course, dropping S1 also increases background. Nevertheless, thanks to self-shielding, the number of events in the center of the detector (blue triangles on the plot above) is small enough to allow for setting strong limits. Indeed, using just 12.5 day of aged Xenon10 data a preliminary analysis shows that one can improve on existing limits for the scattering cross section of a light dark matter particle:Most interestingly, the region explaining the CoGENT signal (within red boundaries) seems by far excluded. Hopefully, the bigger and more powerful Xenon100 experiment will soon be able to set even more stringent limits. Unless, of course, they will find something...

Random collection of final impressions, and a tentative balance

2010-07-30T11:10:00.006+02:00

ICHEP is over. After the last plenary session the few remaining braves stormed out of the auditorium, strained with conference fatigue, and headed back home. I must confess, I found a week-long conference, with six full days packed with presentations, pretty long and tiring. I'm not completely surprised that in the last days not many questions came from the (depleted) audience.

Since this is probably my last entry in this blog, I'll entertain you with a random collection of final impressions, and maybe a tentative balance on the blogging experience itself.

The conference itself

Lot has already been said and written, so let's simply put it this way: the conference was excellent. Superb location (Paris is always Paris), excellent venue (I was just astonished the Palais de Congres doesn't provide wireless microphones in the smaller rooms, everything else was perfect), very efficient organization (thanks!), and an optimal balance of contents. Ok, the catering was less-then-perfect, but why should we indulge in complaining about the little details? :-)

The LHC has entered the game

Again, not a big news, but it's good to repeat it: we begin to see the first physics results from the LHC experiments! And even if this is not yet exciting new physics, those times are approaching fast: after more than 20 years of preparation, it's a nice sensation for the whole community.

Experiments vs theory

On the low side, I must say that I found the theory contributions in the first part of the conference a bit isolated. This is probably normal in the context of parallel sessions (and there were anyway good phenomenological contributions in the sessions more oriented to experiment), but as an experimentalist I probably missed the opportunity to learn something really new for me. For instance, I learned from Georg that:

the talks in the lattice session had actually been selected to be accessible and of interest also to people outside the lattice community (in particular there were a number of review talks), so it was a bit of a pity that the talks were attended almost exclusively by lattice theorists.

I agree: pity! Maybe this should have been advertised more? The situation was of course different in second part of the conference, and I really appreciated some of the more theory-oriented talks in the plenary sessions.

"Sliduments" vs nice talks

The quality of the talks was in general rather good, and of course touched its best in the plenary sessions. I had anyway the impression that the non-LHC and non-Tevatron speakers gave the best talks in the parallel sessions. I have a theory, at least for the LHC talks. Nowadays we (the LHC experimental physicists) routinely use slides as a support for documentation of the daily work we are doing. Most of us have taken the (bad!) habit of packing them of all the information we want to record, information that should anyway go into a written report, sacrificing the graphical quality - and the effectiveness when used as a visual support for an oral presentation - in favor to an hybrid object that the experts in the field call slidument. Sure, it's possibly something easier to present: one can pretend to use the text on the slide as a reminder of what to say, maybe even avoiding to reharse. Well, the quality of this kind of presentation will definitively be worse, it's guaranteed, and - if they maybe can fit a weekly collaboration meeting - will certainly not meet the standard needed for a conference . Have a look at the slides of some of the presentations in plenary session of Wednesday, for instance the ones on dark matter or cosmology:

Almost no text, just the few word need to stress the concept, clear figure, no clutter. Sure, the speaker must now what to say on this slide! Now compare for instance with this one (taken from an ATLAS talk, so that nobody can say I'll try to blame our competitors only):
No excuse, we have still a lot to learn!

Blogging ICHEP 2010

I am still digesting the experience, and in this sense I'd really appreciate to get some feedback by the readers on this. On my side, I can say it has been interesting to blog a conference - it was a primer for me - and to do it in a collective blog, with different voices and styles.

Some of the feedback I go tell me that the blog has been appreciated outside, especially by the colleagues that were not attending the conference: apparently it helped to feel connected, more than the webcasts and slides only can do. It might also have helped the journalists reporting the conference to the media: a blog like this can certainly act a filter, and help the non-physicist to grasp what's important, what gets us excited, and why.

This (semi)official blog of the conference was an experiment, and in this respect the organizers wanted to keep a low profile, and verify on the field what the reactions would have been. It seems to me that, if in effect the community seems interested by the format, maybe next time something slightly more ambitious could be tried. For instance, with a bit more of organization we could have had some video interviews at the conference (someone did that, and did it very well indeed), a dedicated Twitter stream, and especially more visibility at the conference itself. I had in fact the impression that - at least at the beginning of the conference - a large part of the participants had no idea that this project was existing at all. And, since the most interesting and useful part of the blogging experience is the conversation with the readers, this could have been even more fun.

Anyway, I would probably do it again, should the occasion came. See you in two years in Melbourne?

The ICHEP Effect

2010-07-30T00:13:00.001+02:00

I created a tool that watches how many plots DZERO, CDF, ATLAS, and CMS release as a function of time. Here are the results for this year (each little square is a plot):

I’m going to call that bump in July there the ICHEP effect.

The CMS Momentum Scale And Resolution

2010-07-29T20:26:00.003+02:00

While the focus of the international conference in high-energy physics in Paris last week has been on the search for new physics and the precise measurement of standard model quantities, I will offer to you today something more technical, but in no way less physics-rich; it was presented in Paris, but with the many parallel sessions it may have well gone unnoticed... What I wish to explain to you is the procedure by means of which the CMS experiments calibrates the scale and resolution of its charged particle momentum measurement.

The dull sound of the topic as stated above should not deceive you: this is a really exciting, interesting technology, which allows the measurement of physical quantities with high precision. Since the M in CMS stands for "muon", we certainly care for the precise measurement of muons -and muons are the particles used for the calibration procedure.

What happens when a charged particle leaves ionization deposits ("hits") in the silicon tracking system is that we can reconstruct its trajectory, forming a track. The track is curved in the plane transverse to the beam, because the S in "CMS" stands for "solenoid", a big cylinder that provides a B= 3.8 Tesla magnetic field within its volume. If you know what the Lorentz force is, you might also remember the formula P = 0.3 B R, expressing the proportionality of the momentum of a charged particle and its curvature in a magnetic field. This demands that within the CMS solenoid a P = 1.14 GeV muon follow a curved trajectory, which resembles a circle of radius R = 1 meter if observed in the "transverse" plane to the beam axis, the one along which the solenoid is symmetrical. By measuring the curvature, we determine the transverse momentum!

Things are always complicated if you want perfection. We of course can measure the position of the silicon hits with extreme accuracy, but alignment and positioning errors may create imperfections in the measurement of the track curvature. We also know the magnetic field with high accuracy, through Hall probes and other means, but imprecisions will affect the momentum measurement. Finally, the amount of material of which the tracking detector is composed affects the trajectory, producing further imprecisions if our map of the material is not perfect.

In the end, all the effects and all the details of the geometry of our detector are encoded in a carefully crafted simulation. With the simulation we can figure out what a 1-GeV track would look like, given our reconstruction and our assumptions about geometry, material, and magnetic field. But we need real data to verify that our model is correct, and to tune it in case it is not!

Real data: we now have it. CMS uses resonance decays to opposite-charge particles for this business: they are easy to identify, have little background, and there are plenty to play with. In particular, we use J/Psi meson decays to muon pairs for some of the checks of the momentum scale and resolution. Other dimuon resonances are also used -there is a large amount of such decays already available in the data so far collected- but here I will only discuss what CMS did with its J/Psi signal.

The dimuon mass spectrum in the vicinity of the nominal J/Psi mass value is shown in the picture below. A large number of signal events is observed. These events can be used to calibrate the momentum scale.

If one looks closely, one observes that the measured mass is very slightly lower than the nominal 3.097 GeV. This is already evidence for a very small underestimation of the momentum scale. To dig further, a simple thing one can do is to divide the J/Psi events depending on the value of the particle's reconstructed momentum or rapidity, measuring the mass in all sub-samples to check if in particular kinematical regions there is a bias. The bias, of course, would arise from the momentum reconstruction of the individual muons; but if one only measures the mass, which is a quantity constructed with the measurement of two muons, surely only an "average" bias can be detected, right ?

Wrong. Each muon from the decay of each J/Psi has a different momentum, travels through different parts of the detector, and is subjected to different reconstruction biases: we can turn these differences to our advantage. What we can do is to assume we know the functional form of these biases, and plug them into a likelihood function.

A further benefit with respect to methods I have seen in the past for the correction of scale biases is that a well-written likelihood function is also capable of extracting the momentum resolution from the same set of data. One just needs to produce a functional form (whose exact shape is suggested by simulation studies) that describes how the resolution on the momentum depends on the track kinematics; then, the likelihood fit will take care of finding the best parameters of the resolution function as well, by comparing the expected lineshape of the resonance with the mass value measured for each particle decay.

The likelihood is very complicated, because it accounts for the dependence of the mass on the muon momenta and the resolutions, and momenta and resolution in turn are functional forms of bias parameters. I know very well the code of this likelihood function, and I can tell you it is not for everybody! So I will abstain for once from finding a suitable analogy, lest I squeeze my brains for the rest of the evening. Let me just say that in the end, the likelihood maximization produces the most likely value of the parameters describing the bias functions, allowing a correction of the bias in the track momentum measurement!

Maybe it is best to show a couple of figures. The first one below shows the average mass of the J/Psi meson as a function of the pseudorapidity of the muons from its decay. The hatched red line shows the true value of the J/Psi mass; but more meaningful are the crosses, which show what should be measured with a perfect detector, given the fitting procedure (which, I am bound to specify, assumes that the lineshape follows a Crystal Ball form). The crosses are our "target": if we measure a mass in agreement with them, given our fitting procedure to extract the mass, our momentum scale is perfect.

In blue you can see that the mass, before corrections, is biased low, especially at high rapidity. Instead, after the likelihood maximization and the correction procedure, we obtain the purple crosses. The agreement with the black crosses is still not perfect, and the statistics is too poor to detect further small deviations, but the demonstration of the validity of the procedure is clear!

And then, the resolution. This is also a function of rapidity in CMS, due to the way the detector is built and the decay geometry. The figure below shows what resolution we expected to measure as a function of rapidity, from simulated J/Psi decays (in black), given the measurement method.

In red the figure also shows what the true resolution is, from simulated muons that are then compared to reconstructed ones. In blue, the band shows what instead CMS measured. The agreement between data and simulation is encouraging, and the result demonstrates the validity of the method. This functional form and its parameters are extracted from the way the reconstructed masses of J/Psi decays distribute around the nominal mass, accounting for the fact that muons in those events have different rapidity: the likelihood knows all the details, and produces a very complete answer to our question.

I think the method is very powerful and I cannot wait to see it applied to all resonances together, with more data -the different dimuon resonances have different kinematics and produce muons of widely varied momenta, allowing a very complete picture of the calibration and resolution of the CMS detector!

Final days, brief summary

2010-07-28T14:22:00.005+02:00

Tuesday was the days of the quarks -- almost every talk was about QCD or flavour phyics.

Unfortunately, the lattice QCD talk did perhaps not make the best of the opportunity to convince a larger HEP audience of the importance of lattice results, since Yoshinobu Kuramashi chose to pass by the many important contributions that lattice QCD can make and has made towards flavour physics, and concentrated on the derivation of nuclear properties from lattice QCD instead. This is clearly a very important topic and replacing phenomenological models with true first-principles predictions from QCD will have an enormous impact on nuclear physics; the present audience of experimental high-energy particle physicists and Beyond-the-Standard-Model theorists might have been more excited to hear about lattice determinations of the decay constants and semileptonic form factors of heavy mesons, however.

The decays of B and B_s mesons are also the field in which the most exciting discrepancies between experimental results and Standard Model predictions keep appearing. While the D_s discrepancy of last year has disappeared in the meantime, there is now some tension in B meson decays, the most significant of which (at 3.2σ) is the like-sign dimuon charge asymmetry measured at the Tevatron. However, combining all results from D0 and CDF gives a discrepancy of less than 2σ when compared with the Standard Model, while all individual results are mutually compatible within errors. So perhaps this is again a fluctuation, or else something really weird is going on here. Phenomenologists may have a clearer picture of what that could be, see the post by Jester.

Today (i.e. Wednesday) started out with a session on neutrinos. Neutrinos have their own version of flavour physics -- neutrino oscillations. The MNS matrix, which is the leptonic analogue of the CKM matrix, is a lot less well-known than its hadronic cousin, however. In particular, it is not clear whether the mixing angle θ₁₃ describing the mixing between the first and third generations is non-zero, although recent results indicate that it likely is. The flavour structure of the neutrino sector might be much richer than the quark one, though, since the existence of sterile neutrinos (i.e. neutrinos not partnered with a charged lepton via the weak interactions) cannot be ruled out at present.

The neutrinos detected at great effort by huge detectors can come from a variety of sources, some of which got their own talks: Alain Bellerive talked about solar neutrinos (i.e. the ones created by nuclear fusion in the Sun), where the Solar Neutrino Observatory (SNO) is now beginning to achieve precision measurements, which favour a scenario of large neutrino mixing angles. Tsuyoshi Nakaya presented long-baseline accelerator neutrino experiments (where neutrinos beams are created at an accelerator from the leptonic decays of a beam of charged particles), among whom OPERA has recently reported the first candidate for a ν_τ oscillation. Fabrice Piquemol spoke about reactor-based experiments (where the neutrinos come from the reactions in nuclear reactors) and single and double β decays; the tail of the energy distribution of the electrons produced in β decays can impose an upper limit on the mass of the electron neutrino (the limit of 2.3 eV does not seem terribly competitive with other limits, however), and the observation of neutrinoless (or "0ν") double β decay would establish that neutrinos are Majorana particles, indentical to their antiparticles.

Today's second session was about the links between particle physics and cosmology: Dark matter is one of the big "known unkowns" in our present understanding of the universe. What is known is that it cannot be normal (hadronic) matter because of limits imposed by Big Bang nucleosynthesis, and that it cannot be neutrinos, either. The best candidate would be a WIMP (i.e. a Weakly Interacting Massive Particle), such as the lightest stable superpartner of a Standard Model particle if SUSY exists in nature. The discovery of such a particle at the LHC would thus have significant impact also on cosmology.

Trouble with Flavor

2010-07-28T11:40:00.006+02:00

Flavor physics gives me a headache. Unfortunately, this sub-field of particle physics is where new particles and interactions are very likely to show up, so it's essential to follow all hints from latest observations. Yesterday at ICHEP Gino Isidori gave a nice theoretical summary of where we stand.

Overall, the standard model frustratingly well explains the multitude of observed transitions between quarks and leptons of different generations. If we extend the standard model with generic non-renormalizable 4-fermion operators, their coefficients are extremely constrained by experiment. The scale suppressing certain flavor-violating operators has to be at least 100 TeV in the bs sector, at least 1000 TeV in the bd sector, and the astounding $10^5$ TeV in the ds (kaon) sector. It means that if any new particles exist at the TeV scale they better be very careful not to destroy the approximate flavor symmetries of the standard model, as otherwise they would generate effective 4-fermion operators with large coefficients.

That probably means that at the TeV scale there is no new particles beyond those of the standard model. There is still some hope, however, that the above is not true, and this forlorn hope is fueled by three results that are currently in tension with the standard model predictions. One is the widely discussed the D0 measurement of the same-sign dimuon asymmetry which points to new contributions to $B_s-\bar B_s$ meson mixing at 3.2 sigma level. The 2 other less widely known discrepancies are:

Various determinations of the beta angle in the unitarity triangle (determined most precisely from $B_d \to J/psi K_S$ decays, and from the $\epsilon_K$ parameter in the kaon mixing) do not agree very well. The current discrepancy is around 2.5 sigma.
The branching fraction of the $B \to \tau \nu$ decay measured by BaBar and Belle is currently two times larger than the standard model prediction. Given the errors, the current discrepancy with the standard model is around 3 sigma.

These are most likely flukes, unavoidable among such a huge number of measurements, but it won't hurt to keep an eye on those.

Theorists have put up several models that may fit all up-to-date flavor observables and explain the existing anomalies. For Gino, the favorite model is the 2-Higgs doublet model. In this scenario, the Higgs sector contains additional 4 scalar particles who can mediate flavor violating transitions. The point is that they do it in a very respectful way, including the suppression by small CKM angles and by small quark masses, so they not to produce excessively large effects even when the new Higgs particles are at the TeV scale. The quark mass suppression leads to the desired pattern where the smalest new contributions come in the kaon sector, while the largest occur in the Bs meson sector. This scenario also predicts new contributions to $B_s \to \mu \mu$ decay and to the neutron electric dipole moment at the level of the current sensitivity, so fresh tests of this idea are soon to come.

For more details, see the ICHEP'10 slides. As a bonus, a very accurate rendering of theorists waiting for hints of new physics in flavor physics.

How can there be no questions?

2010-07-28T10:32:00.001+02:00

I’m sitting here, on the last day of ICHEP, listening to some excellent summary talks. One of the experimental neutrino talks just went by – excellent (sorry, no link because I don’t have internet as I write this). But there were no questions. I’ve noticed this for many of the talks at the plenary. Or if there is a question, it is the “if you doubled your dataset what would happen to that 2 sigma excess?”

How can this be? There are almost 1000 people watching these talks, many of them experts in the topics being discussed. And no questions? Physics is built on questions! This is how we learn – almost never is something so clear that we don’t need questions! Heck, the whole field is structured around this. We invite experts to our University to give a seminar so we can have their undivided attention for a whole day to ask them questions. We go to workshops so we can ask each other questions and learn. We write papers and then respond to the papers with letters and more papers which are basically a slow version of Q&A. So what is going on here?

I don’t know, of course, however I’m going to take a stab:

Exhaustion. It is the end of a full week of conference. We’ve all be talking, discussing, and thinking about these topics for a solid week and we need a break for our brains to process everything.
Embarrassment. There are 1000 people here. Many of them very important people in the field (i.e. they think you are stupid and your chances of funding go down). So you’d better make sure you have a good question before you ask it.
These are summary talks. The speaker is often presenting a huge amount of information in a very short amount of time. It is likely they are an expert in the topic they are presenting, but given the volume of information discussed in such a short amount of time, it is not likely they know all the details. The people who are experts on each topic gave talks in the plenary sessions. Indeed, there were lots more questions there than there are in the plenary sessions.

I’m sure there are other possibilities as well.

Is there?

2010-07-27T17:50:00.002+02:00

I just liked this...
For better pictures see Marco's photostream!

Back to the future

2010-07-27T15:42:00.005+02:00

It's Tuesday and I'm exhausted! I've only just found some time to sit down and think about the future session that happened way in the past (on Saturday, pre-Sarkozy, pre-press conference, pre-dinner - gee, was it only three days ago?!). Anyway, with the network problems already reported by Gordon, the dinner already covered by Katie and hundreds of reports in media around the world I hope I am excused. Also, sitting in the press booth isn't exactly supportive of concentrated work as many people pass by for a chat. I guess I have to educate them to pass by for a chat AND bring a coffee. Since you ask, no sugar but a lot of milk please, thanks!

But now, here goes! Saturday saw a big overview of all the exiting and less exciting future projects for the world of particle physics. I leave it up to your judgement to decide which is which, comments are welcome! I am also arranging them by my brain's personal sorting system and will happily accept comments and corrections - the list is probably not complete.

Particle physics is a global field. You just have to look around around the room to notice that people come from all over the place. The big machines that we work on these days are challenging and cost a lot of money so that no one country could afford to build and host them - all countries have to chip in and work together. The more challenging the technologies become the more this is the case, and it also takes many years for a machine to evolve from idea to design study to running accelerator. Consequently it may seem strange to the outside world that while we've only just switched on the LHC and are waiting for discoveries, we already plan the next generation – but we have to have a variety of options in the drawer that will enable us to make the best choice when results are there. And then of course there are physics topics that aren't covered by the LHC!

There are several ideas for 'LHC follow-ups'. Two different varieties of LHC upgrades exist - one for more luminosity and thus higher statistics and safer discoveries, one for higher energies. Whereas one, the luminosity upgrade, is virtually around the corner, the energy upgrade is an option that's far in the future (around 2030, according to Roger Bailey's talk from Saturday). The discoveries at the LHC will probably dictate whether the higher energies are more interesting, or whether the LHC could be transformed into a electron-proton machine, or a 'superHERA', although it goes by the name of LHeC in the session, with the e for electron. LHeC would collide the LHC's protons with electrons from a linear collider - an intriguing thought for someone working on the ILC! My imagination already went off into dreamscapes where LHeC and the linear collider would run together on different physics programmes as the best possible synergy of machines we have yet to see (and believe me, physicists are great at creating synergies and reusing existing machines!). I guess I'll have to talk to a few proper scientists to check whether this is imagination running wild or whether it's actually possible.

Certainly possible and the most likely next big project is a linear collider for electron-positron collisions. It'll complement the LHC and it's only a question of LHC discoveries, again, whether the ILC or CLIC is the machine of choice. While the ILC is basically ready to be built - the Technical Design Report is due in 2012, which means "Here's how we would build it", CLIC is a few years behind, with its Conceptual Design Report due next year. When you think that first collisions from a linear collider could be expected in the 2020s you start to understand why there is plenty of planning, designing and testing going on around the world!

Then there are b factories, machines that would complement and extend anything that the b physics experiments around the world, like the LHCb experiment at the LHC, find. One is proposed in Italy, and KEK in Japan has just started reconfiguring its KEKB accelerator into - can you guess its name? - SuperKEKB. Funding isn't final but they are planning get 40 times more luminosity. The Italian b factory would also be a light source, and it shares its multifunctionality with Fermilab's 'Project X', which can contribute to the ILC, to a possible muon collider and ultimately a possible neutrino factory. Fermilab is busy working on a plan for a muon accelerator program (i.e. a MAP), and muon colliders, though technologically still a big challenge, are also a big topic for machine physicists. Probably something called 'dielectric acceleration is, too, but I couldn't tell you as I didn't understand the talk, sorry....but when asked about whether there is a plan for a beam delivery system, the speaker laughed and said that he'd like to have this questions again in 10-15 years -- so I conclude it's not something that would pop up in the next months.

What I missed in almost all of the talks were good, catching, convincing arguments why these machines that were proposed are needed. I am sure there are solid physics cases for all of them, but surely it can't harm to state them again, clearly, understandably, in talks like these that will live on for a while? I'll go hunting for them for a future story in NewsLine, but first I go hunt particles in the Grand Rex, the nuit des particules -- see you there!

A Spectroscopist's Delight!

2010-07-27T15:31:00.005+02:00

While everybody is busy discussing the latest Tevatron results on the Higgs boson searches -is that the light-mass excess the internet was abuzz, is it consistent with a signal as we expected it, how long will it take to confirm it is not a fluke, etcetera, etcetera, etcetera- I think I have a different plot with which to enthuse you.

If you do not like the figure below, courtesy CMS Collaboration 2010, you are kindly requested to leave this blog and spend your time reading something else than fundamental physics. I do not know what will ever make you believe particle physics is beautiful, if not what is shown here.

The figure shows, using a logarithmic scale on both axes, the reconstructed mass of pairs of muon candidates of opposite charge, collected by CMS in its first 280 inverse nanobarns of 7-TeV proton-proton collisions collected until a week ago. Nothing fancy has been done to prettify this graph: these are honest-to-god muon pairs, as Nature (the bitch, not the magazine) has produced them in the core of CMS. True, the interecession of a detector and a reconstruction software were needed to go from ionization clouds to event counts; but this is the absolute minimum of manipulation you can ever expect from particle signals.

Now, what should enthuse you about the graph is the following. The distribution reveals, clearer than a million words could describe, the structure of all the most important bound states decaying by electroweak interactions into pairs of muons which we can produce in hadron collisions. We immediately spot the Z boson on the far right, and the towering peak of J/Psi mesons; but we also see Upsilon mesons, and at lower energy, we detect the ligher resonance decays of rhos, omegas, and phi mesons. What a spectroscopist's delight! This figure is tremendously informative! If we sent it to outer space, without labels or units, no intelligent race could ever mistake its meaning!

You also notice that these jewels stand atop a background of unidentified muon pairs. Muons can be produced singly by the weak decay of kaons and pions, for instance, or even more massive states like bottom and charm. Occasionally, pairs of muons of opposite charge can emerge that do not have the same parent: the frequent production of these uncorrelated pairs creates the significant backgrounds you see in the picture. Note, however, how these backgrounds die out for large dimuon masses: the Z boson is basically background-free, a fact I have noted in my previous posting here.

As these pages testify, CMS and ATLAS have presented scores of interesting physics results at ICHEP this week. None of those were groundbreaking ones; a few were significant advances, though, and many others were just meant to demonstrate that the experiments are ready for big challenges, such as discovering new physics, the Higgs, measuring the top mass better than the Tevatron, etcetera. The presented results took about a hundred man-years to produce, and I have a lot of respect for them -not to mention the fact that I did my little bit to contribute. But it is my humble opinion that the graph shown above could well be the one to single out and attach on the bulletin board of all the universities and institutes participating in the LHC experiments!

Paris est la France, et la France est Paris

2010-07-27T15:23:00.009+02:00

If you have lived in France (but not in Paris) at least for a little while, you have certainly learned the hard way that there's no France outside Paris. Or, like the inhabitants of the City of Light prefer to put it, Paris est la France, et la France est Paris.

While there are certainly good reasons to identify the French grandeur with the capital city, there is certainly more in France that just Paris. And I'm not only thinking about the food variety, that we experienced yesterday evening at the conference dinner where the excellent food specialties of five different French regions were served. Actually, that we should have experienced, and that were supposed to be served: in reality the food flow was definitively not enough to satisfy the appetite of the disappointed conference participants. But this is another story.

This valuable variety (of fine food, nice places, and of other excellent things) is rarely recognized by the Paris people - that's a fact - but it is was rather unfortunate to discover that this point of view seems to be shared by the French President himself. As you might have heard, Nicolas Sarkozy attended the ICHEP conference yesterday to give an "opening" talk (well, the conference was opened since four days, but still). Now, I really don't want to indulge in commenting the speech, nor I dare to discuss the subtleties of the French research politics. But I cannot refrain at least to note (at a very superficial level indeed) that both in the announcement of the President participation, and especially in the President speech, the French excellence in high energy physics seemed to be contained in circle of a 30 km radius centered on the Tour Eiffel.

I can tell you, this has definitively not pleased those ATLAS colleagues of mine working for instance in Annecy or Marseille, and I guess the other French physicist working for instance in Grenoble, or Clermont-Ferrand, or Strassbourg must not be that happy too. Oh, yes, the press office of the Elisee has finally changed the initial announcement, a now they are cited along with the other Paris university and research centers as "universités de province", but I'm still not sure they are at ease with the classification. And, despite the last-minute change on the web site, the President speech itself was still confined to the "region parisienne"! Are the Paris people really so distracted? I would tend to doubt it, but again, this would lead me to speculate about the current French research politics, and I'm certainly not qualified for this. Pity, anyway.

Physicists gone wild

2010-07-27T11:13:00.004+02:00

Most (perhaps all?) physics conferences share the conference dinner tradition. One evening during the conference, all participants are invited to get together to share a meal, usually featuring cuisine from the local area. Last night was the ICHEP banquet (the fancier term ensured that quite a few attendees came in fancier attire than they wore to the sessions). The banquet took place at the Grande Galerie d'Evolution, and while the gallery, stuffed and mounted wild animals, and line to get in and check our laptop bags was certainly grand, the food and drink were another story. The crowds surrounding the tables holding tapas-style food from various areas of France wouldn't have looked out of place on a rugby field.

(Here's a photo of physicists in the wild, during the calm before the food-induced storm.)

But the point of conference dinners isn't really food, but to talk to old and new colleagues, and that went off without a hitch. The food situation also provided a perfect ice-breaker for conversation, and I heard many a story about banquet disasters from conferences past.

Tonight will see a gathering of a very different sort in Paris: the Nuit des Particules, a particle-physics-themed evening for the public. The event at the Grand Rex theater in Paris starts off with a public lecture by scientist Michel Davier (in French), at which French actress Irene Jacob will also be present, and ends with a showing of the science fiction film Sunshine (in English).

Day 4: the Higgs is not there. Yet.

2010-07-27T09:56:00.021+02:00

Now we know, the Higgs boson did not show up at Tevatron. Yet.

But we also know that, if it exists, we would not find it in the 158-175 GeV mass range. The saga of the Tevatron Higgs talks finally came to an end, certainly matching the expectation, at least for what concerns the show part. Well, as for the scientific part, after all the preliminary steps of the last days nobody was really any hint of signal anymore. We mortals might not be able to make fancy statistical combinations by eye, but it was still not too complicated to anticipate that, since there was no excess in any channel or single detector combination, we should not have expected any dramatic announcement.

I found Ben's talk really excellent: the slides (that, by the way, were kept secret until the very last moment) were very well prepared, and Ben proved to be an excellent presenter. In a sense he was even rather humble, especially if you think about the fuss about a possible signal before the conference (at slide 46: "I'm sorry, this 2 sigma excess is the closest we have to a discovery"!).

Still - but maybe it's just me- I had the feeling that the presentation contained a subtle innuendo. Of course Ben did not dare to say anything that was not scientifically backed, but take for instance slide 36. Ben gently dropped this CDF plot:

and casually commented: "People keeps on asking how would the exclusion plot look like if there existed a Higgs boson. We did the exercise of injecting a 115 GeV SM Higgs boson signal in several channels, and that's what we got". And, guess what, the results is that the exclusion curve jumps up like it had a 1 sigma fluctuation on a rather large mass range. Now, doesn't this jump remind you of any feature we saw in another curve? There a region where the unspeakable dreams and hopes of many live, between a green and a yellow band.

Monday, brief summary

2010-07-27T09:15:00.001+02:00

Since I was having trouble with the WLAN yesterday, I couldn't blog the first plenary session. My colleagues have already written in more detail about the most interesting results presented, so I'll just give a brief summary of my impressions: The LHC experiments are well on their way towards "rediscovering" the Standard Model (they have the low-mass resonances, the J/ψ, Υ, W and Z, and several top candidate events). Nicolas Sarkozy is a politician; it is of course a great honour to have him officially open the conference (in particular when he calls the participants "the hope of the planet"), but it might be unwise to put too much faith into his words regarding research funding. he Tevatron has excluded the Higgs mass range of 158 to 175 GeV and is beginning to also exclude a low-mass Higgs (although the LEP bounds are still stronger there). The rumour about a possible discovery of the Higgs as the Tevatron was just a rumour, the 2σ fluctuation observed is not significant.

A significant discrepancy between theory and experiment was, however, observed at the banquet: theory predicted a stable circulating beam of physicists scattering elastically off several targets representing the culinary traditions of the different French regions. Experimentally, a lot of pile-up events were observed, leading to early depletion of the targets and a failure to achieve saturation for many participants ... clearly, some more effort needs to go into the modelling of such high-density situations in culinary physics.

Snapshots from Monday plenary session

2010-07-27T01:24:00.006+02:00

Featuring Steve Myers and the status of LHC, the Spokespersons of the LHC experiments and their highlights, physicists queuing for the Presidential speech, Nicolas Sarkozy at ICHEP, the new Tevatron Higgs combination. Have fun.

The present and future of the LHC

2010-07-26T18:23:00.003+02:00

Today's been a hectic day: between Sarkozy's speech, the press conference, and the lack of wireless internet in the auditorium where the plenary talks are being held, there's been little time to blog. So any time I've been able to grab during coffee breaks has been spent uploading blog entries to my usual haunt, symmetry breaking. There you can read about the 10-year plan for LHC running and whether or not it might affect some Tevatron scientists' hopes to run their accelerator for a further three years. Plus my take on the new measurements reported by the LHC experiments at ICHEP, such as the W cross section measurements at 7 TeV, and the limits ATLAS and CMS have placed on some exotic physics. (Keep in mind that symmetry is for a more general audience; my fellow ICHEP bloggers have covered those results in much more detail here.)

A fun way to browse ICHEP talks

2010-07-26T17:57:00.001+02:00

One of my hobbies is better ways to visualize information with modern computers. Those of you that have followed me know about my efforts with DeepZoom. So, ICHEP is being rendered this way as we speak.

If you click on the link above your browser will load up a very large image using Sliverlight (which you’ll need to have installed: Windows and Mac computers only, I’m afraid). You’ll see about 400 talks all displayed at once. You can then use your mouse and mouse wheel to zoom in to see a particular talk. Your browser should load up the bits of the talk you are looking. You can click on a slide to bring it full screen and use the back and forward arrow keys to move around.

BUT: you need a decent computer, decent graphics card, and, above all, a decent internet connection. In short, no one at ICHEP can look at this display because the internet connection is not robust enough (I’d add a picture here but I’m at ICHEP and can’t load it up right now!).

The display is updated about once every 4 hours, so as the plenary talks continue they should slowly show up there in that display.

Enjoy!

Higgs still at large

2010-07-26T17:39:00.011+02:00

Finally came the moment we all waited for at this conference:
Tevatron now excludes the standard model Higgs for masses between 156 and 175 GeV. The exclusion window widened considerably since the last combination. Together with the input from direct Higgs searches at LEP and from electroweak precision observables it means that Higgs is most likely hiding somewhere between 115 and 155 GeV (assuming Higgs exists and has standard model properties). We'll get you bastard, sooner or later.

One interesting detail: Tevatron can now exclude a very light standard model Higgs, below 110 GeV. Just in LEP people missed it ;-) Hopefully, Tevatron will soon start tightening the window from the low mass side.

Another potentially interesting detail: there is some excess of events in the $b \bar b$ channel where a light Higgs could possibly show up. The distribution of the s/b likelihood variable (which is some inexplicably complicated function that mortals cannot interpret) has 5 events in one of the higher s/b bins, whereas only 0.8 expected. This cannot be readily interpreted as the standard model Higgs signal, as then one would also expect events at higher s/b where there is none. Most likely the excess is a fluke, or maybe some problem with background modeling. But it could also be an indication that something weird is going on that does not fit the standard model Higgs paradigm. Maybe upcoming Tevatron publications will provide more information.

A Big Day

2010-07-26T17:18:00.001+02:00

I was going to post this before anything started this morning, however the ICHEP committee decided not to purchase wifi in the main conference hall, so it took until now for me to have time to post this….

Today is the first day of plenary talks at ICHEP. Up to now it has all been plenary sessions – and there have been a lot of talks. Now everything collapses into one room. And it is a very big room. I don’t think I’ve ever been in a room this big for a physics conference before.

Several things happen today – but two I’m particularly interested in seeing are the final combination of the Tevatron Higgs searches and a speech by the president of France, Sarkozy.

Sarkozy's speech was much better than I was expecting! He and his speech writers had worked together to craft a quite good message. One part was a vigorous defense of why it is a good idea to invest in science now even though there is a financial crisis on. Basically, his message boiled down to: a country can not ignore the future for the crisis of the present – there is always a crisis of the present. A message I wish more of the legislators in the state of Washington would get. The second half of his message was the details of the investments that France is making. You can’t always trust the numbers that politicians give you in settings like this, but one stuck out: a reinvestment in Saclay – one billion Euros in the next 10 years.

The second thing I’m looking forward to are the combined results from the Tevatron. You can guess what will happen when you look at the CDF and DZERO higgs talks (which I’d link to if there was any wireless at all here!). Both experiments are basically excluding the region around 160 GeV on their own, and both have a downward fluctuation at low mass.

Update from the Higgs talk:

Ben’s fashion choice: Tour de France yellow tee shirt along with a suit jacket. Nice!

High mass region: from 158 – 175 GeV is now excluded (about x4 bigger than before).

Low mass region: Starting to exclude the really low mass region as well now. Expected limit is around 1.2 or 1.4 or so. But the exclusion right now looks like a fluctuation low, so it will be very interesting to watch the next update 6 months from how. The expected is getting quite close to expected SM cross section! Sweet!

Sunday impressions

2010-07-26T13:44:00.005+02:00

Sunday was the day off between the parallel and plenary days. I used the free time to walk into Paris from my hotel in Suresne (a very nice way through the Bois de Boulogne) and through the city before spending the afternoon in the Louvre (which is of course way too big to explore even in a whole day, so I just took a look at some of the Oriental Antiquities, Dutch paintings and Italian paintings -- there is so much more beauty there than "La Gioconda" to whom everyone rushes without looking around them).

I was quite impressed by the diversity and level of the street music in Paris once again: at the Pont Neuf there was a man playing Handel's Music for the Royal Fireworks ... on a steel drum! Not exactly an orthodox instrumentation, but listening I thought il bravo Sassone would have been delighted by the adaptation. And just a little further, in the Châtelet métro station, there was a string septet playing Mozart.

Now for Monday and the presidential speech.

Electroweak Signals From CMS

2010-07-26T13:34:00.002+02:00

The ICHEP parallel sessions are over, and it is time for a summary of results. Of course if you are in Paris you will get it from the summary talks, but if you prefer some armchair, remote attendance of the conference, I have collected for you a few meaningful plots.

Here I wish to assemble some of the electroweak physics results produced by CMS in time for ICHEP. The CMS experiment has shown results that use up to 280 inverse nanobarns of proton-proton collisions, but for electroweak measurements -those involving W and Z signals, to be clear- the statistics used is up to 200 inverse nanobarns of well-understood data.

It is exciting, and quite pleasing, to see how quickly the results have been produced. The speed at which a collaboration goes from raw data on tape to plots for conferences is in my opinion a quite important indicator of the confidence of the collaboration on the whole chain -detector, analysis tools, internal scrutiny. And CMS appears to pass this evaluation with flying colours!

So, W bosons are readily produced in proton-proton collisions, as is clear in the distributions below. These show the transverse mass of muon-neutrino systems, in events where a high-momentum muon has been detected, and where the calorimeter is used to measure the imbalance in the energy flow due to the escaped neutrino.

From a comparison of the left and the right panel you can see that LHC produces more positive W bosons than negative ones (the W contribution is the yellow histogram in both panels, turning on above 40 GeV of transverse mass). Violation of some basic symmetry rule ?? No, simply the result of the initial state containing more positive-charge quarks than negative-charge ones!

Please also note how clean the W signal is. These distributions will one day allow us to improve the already excellent precision in the mass of the W boson, plus to perform a host of other detailed studies.

One thing we can do already now, however, is to study the energy of jets recoiling against the W boson. This can be seen in the plot attached below: the recoiling jet transverse energy follows closely the predictions of simulations.

And what about Z bosons ? Well, of course they are less frequent -because of the smaller production rate, and because of the smaller branching fraction to electron and muon final states. Still, CMS produced significant signals already with 200 inverse nanobarns of data. Have a look at the dimuon mass, shown both in linear (left) and logarithmic scale (right) in the figure below.

What is significant in the plots is the extremely clean signal these decays provide: backgrounds are totally invisible in a linear scale, and they only appear in the log plot. At the Z mass peak, backgrounds appear to amount to less than a part per mille. Also worth noting is the very good resolution of the detector: the width of the Z boson mass distribution is close to that which the Z naturally has, due to its extremely short lifetime.

A similar signal is visible with electron-positron final states, as shown below:

Again, one notes the extremely clean nature of these events: the QCD background is mostly irrelevant. However, this is an inclusive selection: if one were to look for events with a Z boson and several jets, say, the QCD component would dramatically increase its relative importance. Such considerations will come into play when we search for new physics signals!

With the data shown above, CMS has measured the cross section of W and Z production in electron and muon final states at 7 TeV, as well as the ratio of W over Z production, a number which can be known with better accuracy than the absolute rate, due to the canceling of several systematic uncertainties. You can find all the measurements in the CMS public web pages. Here I will just flash one last figure, which amiably shows the increase of the production rate of vector bosons with the center-of-mass energy of the hadron collision.

Note that the blue lines showing the trend of cross section versus energy are broken: having proton versus antiproton or proton versus proton changes the production mechanisms, and thus the rate cannot be strictly compared with the Tevatron and UA1/2 measurements (on the left).

All in all, a rich bounty of measurements, already with 200 inverse nanobarns of data! I drool at the thought of what we will do with three orders of magnitude more data next year!

Snapshots from Saturday sessions

2010-07-26T00:14:00.009+02:00

The Fermi Sky

Not everyone is visiting Paris (but still...)

A supernova exploding in Salle Maillot

Lonely poster session

The trouble with Ultra High Energy Cosmic Rays

Preparing for the President...

Day 3: jets!

2010-07-25T23:34:00.013+02:00

Saturday sessions were not really well attended. if we exclude the one on CP violation, CKM and Rare Decays that succeed in packing a lot of people in one of the smaller rooms. Maybe the average ICHEP participant decided to make the grasse matinee, or simply to be a tourist in the City of Light in a very nice and sunny day. Still, in the semi-empty rooms there were lots of interesting things to be learned.

For instance, I was eager to attend the session on Perturbative QCD, Jets and Diffractive Physics: partly because some of the results I have been working on in the last months were presented there, partly because I wanted to know how CMS was doing on the same subject, but mainly because I wanted to know how the LHC experiments are doing on the jet measurements. Jets are in fact copiously produced in the 7 TeV LHC collisions, and even with the not huge amount of data we have collected up to not, ATLAS and CMS are in fact already capable to make nice measurements, and, in a sense, already unique ones.

I was not disappointed: Tancredi, that was presenting the jet measurements for ATLAS, opened his talk with a nice historical reminders: there was a time, nearly 30 years ago, when jets were seen for the first time at an hadronic collider (and presented in Paris!). Those were days when a physicist could get excited for a a di-jet event with a 140 GeV invariant mass, produced in hadronic collisions at a center of mass energy of 540 GeV. Today the hype is about di-jet events of 2 TeV invariant mass: it seems to me that such a comparison helps to put things into a humbling perspective, reminding us how much road has been done, how mush is still to do, and that we are all standing on the sholders of giants.

Both ATLAS and CSM had impressive first cross section measurements for single jets and di-jet objects, already binned in different rapidity regions, and up to unprecedented di-jet masses. And the agreement with the NLO QCD theory calculation is already impressive, despite the data uncertainties are not yet the best possible!

In this particular respect, I was not completely satisfied of the way the CMS explained their approach to get the 5-10% jet energy scale they claim. They certainly have several ratio measurements that reduce the impact of the systematic uncertainty on this quantity, but I'm anyway still curious! And since the data uncertainty is still the dominant one for the cross section measurement of both experiments, and it's mainly driven by jet energy scale, it's a point that will become very relevant as soon at the statistics will be large enough to make precise measurements in previously unexplored $p_T$ and $m_{1,2}$ ranges. This moment is certainly not far in time!

Of course pQCD is a nasty beast, and as soon as one starts to compare his jet results with some tune of his preferred MonteCarlo, he can be assured that someone will ask how much the chosen tunes are reliable, how well they fits with the low energy data from previous experiments, how well he know the MC authors... :-) I suspect a human component in this aggressive questioning: like it or not, jets are really the only domain in which the LHC experiments have already overtaken the Tevatron analyzes in mass reach. Thing that both the speakers did not fail to remind to the audience, and that might have not pleased everyone!

Away from Palais des Congrès ?

2010-07-25T12:11:00.006+02:00

As usual, when you attend a conference, you tend to forget that there are other things happening and other people living outside from the venue of the conference. This is particularly true when you are abroad, away from the daily routine, and the time and attention spent in the session rooms makes you forget where you are, and whether it is morning, afternoon or night...

However, even if you are not around Palais des Congrès now, you can follow all the talks on the webcast of the conference here. This is true for the plenary talks of next week, which will be broadcasted live, but you can also access all the parallel talks that took place in Salle Maillot last week.

In case you want to enjoy a bit your time in Paris (or skip a session, but shhh...), you may still bump into ICHEP 2010 during your walks. Indeed, if you stroll along the Seine and enjoy Paris-Plages, you will certainly notice a stand for kids around physics -- with a "trombinosquark" and other fun activities...

And on Tuesday evening, if you enjoy the atmosphere of the "Grands Boulevards", you will again see another ICHEP-related event. But this time, we have moved from Palais des Congrès to the Grand Rex theatre hall, with the "Night of the Particles" : a popular physics conference and a good movie ! Another way of enjoying ICHEP away from Porte Maillot...

End of Part One

2010-07-25T11:18:00.007+02:00

Sunday... Time for a rest for the participants... as well as for the organisers. Obviously, the last three days were quite hectic for me, since I had both kinds of duties (hard to explain to sa colleague that you see only once every two years that you must post first then discuss physics afterwards). Thus it was not really blogging time for me, as we had a few things to sort out here and there.

As expected, I followed quite extensively the flavour-related sessions. Obviously, now that Babar and Belle have intensively studied the Bd and B+ mesons, the Bs meson is the focus of all interests -- which explains the large crowd gathering in the small room. D0 reported again their exciting results shown at ICHEP for the dimuon asymmetry, which rely on a clever cancellation of some of their background by combining the dimuon asymmetry with the single muon one. As reported by Jester, things are getting far less optimistic -- if you like New Physics -- for the phase in Bs mixing, since CDF agrees now with the SM, whereas D0 remains in muddy waters. Let us hope that both collaboration will provide an average very soon...

Fortunately, the other sessions were less overcrowded, but still quite lively. Belle has showed its ability to run nicely at the Υ(5S), yielding interesting results on some nonleptonic decays for the Bs mesons after analysing only 1/5 of its sample. This makes all the more interesting the prospects of a super-B factory and will probably renew our interest in old-fashionned (but still so reliable) studies on SU(3) and U-spin symmetries. On the lighter side of CKM, the KOTO experiment seems to be right on track, with a completion of the detector in 2011 to reach a sensitivy around 10^-9 on Br(K⁰->π⁰νν-bar) in 2012. Considering how well we understand this mode from the theoretical point of view in the Standard Model, this is excellent news. There were also preliminary results from MEG on μ to e conversion: a few events can be seen in the signal region -- still compatible with no signal at 90% CL (but the best fit result corresponding to 3 signal events, which makes you dream that maybe, with an increased statistics...).

I popped into other sessions to hear a few interesting talks, like the first results from LHCb (with the first measurements of b-bar production cross-section, as well as the first W and Z events), the not-so-surprising non-discovery of the Higgs at CDF/D0, and the related (and maybe underestimated) theoretical uncertainties on the channels used for this search. Unfortunately, our "presidential" change of schedule kept me quite busy out of the session rooms at the time of the new results on neutrinos and cosmic rays, but I have still the plenary talks to learn about them...

Let Part Two begin !

D0 says: neither dead nor alive

2010-07-24T16:30:00.003+02:00

This year CP violation in the Bs meson system has made the news, including BBC News and American Gardener. The D0 measurement of the same-sign dimuon asymmetry in B decays got by far the largest publicity. Recall that Tevatron's D0 reported 1 percent asymmetry at the 3.1 sigma confidence level, whereas the standard model predicts a much smaller value. That would suggest a new source of CP violation, perhaps new heavy particles that we could later discover at the LHC.

The dimuon asymmetry is not the only observable sensitive to CP violation in the Bs system. Another accessible observable is the CP violating phase in time-dependent Bs decays into the J/ψ φ final state. In principle, the dimuons and J/ψ φ are 2 different measurements that do not have to be correlated. But there are (not completely bullet-proof) theoretical arguments that a large deviation from the standard model in one should results in an observable deviation in the other. This is the case, in particular, if new physics enters via a phase in $M_{12}$ (the so-called dispersive part of the mixing amplitude, as opposed to the absorptive part $\Gamma_{12}$), which is expected if the new particles contributing to that amplitude are heavy. The previous, 2-years old combination of the CDF and D0 measurements displayed an intriguing 2.1 sigma discrepancy with the standard model. CDF updated their result 2 months ago and, disappointingly, their results seems perfectly consistent with the standard model. D0 revealed their update today in an overcrowded room at ICHEP. Here is their new fit to the CP violating phase vs. the difference of the widths of the 2 Bs mass eigenstates
Basically, D0 sees the same 1.5 sigmish discrepancy with the standard model as before. Despite 2 times larger statistics, the discrepancy is neither going away nor decreasing, leaving us in the dark. Time will tell whether D0 found hints of new sources of CP violation in nature,
or merely hints of complicated systematical effects in their detector.

Searching for Higgs; finding colleagues

2010-07-24T15:32:00.005+02:00

The hot topic at ICHEP is definitely the Higgs search (even President Sarkozy has caught it!). I just posted a few more Higgs-y details to symmetry breaking. Here I'll discuss something (slightly) less massive--the interactions between physics bloggers.

While a few of the bloggers knew each other (or of each other) before we joined the ICHEP effort, most of us hadn't met in person. So we gathered yesterday for lunch, to discuss blogging, rumors, and bagels. (Our attempt to eat at a French brasserie was thwarted by dozens of other particle physicists, so we ended up enjoying a very American lunch at the Westside Cafe - something especially appreciated by this expat American, who can't find bagels anywhere near Geneva.)

See if you can spot your favorite ICHEP blogger... It's a lot easier than finding the Higgs!

Day 2: Higgs at Tevatron, Higgs at LHC, Higgs on the BBC

2010-07-24T11:59:00.002+02:00

Friday afternoon I sat again in a relatively packed room for the Higgs session. The effect of the rumor seems to fade away, but there is still quite a buzz around the Tevatron Higgs searches, especially because our friends are professionally distilling their results at a tantalizing pace, and - in case you missed that - the final Tevatron combination will be shown only on Monday :-)

Every Higgs search channel at Tevatron in has its peculiarities and its reasons of interest (since I'm working on the $H\to\gamma\gamma$ search at LHC myself, I was for instance particularly interested by this presentation), but what that I always find impressive in these session are not the single analyzes, but the combination of them.

What is rather clear in fact is that neither CDF nor D0 have enough sensitivity and data to see the Higgs (or claim it does not exist) in a single decay channel. Take for instance the $\gamma\gamma$ channel I was mentioning before: with this channel only both the Tevatron experiments can today only place a limit on the Standard Model Higgs boson production around 20 times the SM cross section.

On the other hand, this channel can add about 5% sensitivity to the combined SM Higgs combinations, and plays an especially important role in the mass region around 130 GeV. Similarly, dozens of other channels can bring their small but important contribution to the global sensitivity. Have a look for instance at the list of Higgs searches that are combined by D0:

or at the impressive combination of the CDF limits for all the channels they are looking at:

Putting all these searches together is an industrial work, with a non negligible effort of standardization of the results format, both by the different analysis teams in a collaboration and by the two collaborations. It's something ATLAS and CMS have to learn to do quickly: as it came out during the session, there already exists a combined ATLAS-CMS effort for the statistical combination of their results, and very recently a first exercise of LHC Higgs results combination was performed, but the road to reach the current Tevatron expertise and organization is still rather long.

But - don't you know? - the Tevatron combination will only be presented on Monday, so let's still stick to the separated D0 and CDF ones. As you can see for the previous plot, CDF was lucky and was able to reach sensitivity to the SM at 165 GeV alone. D0 was slightly less lucky, but is nearly there too: congratulations!

The most interesting questions to the Tevatron experiments during the session were all rotating around the same subject: how much more data wold they need to bring their curves below 1 along all the mass range? It's certainly a very relevant question: as you can see form the the ATLAS and CMS talks at the same session, the LHC experiments will need time since we can reach similar sensitivities, and in the meanwhile the Tevatron would certainly like to keep on taking data as long as possible. This is such a hot subject these days that it has percolated to the media, as the D0 speaker reminded us:
It's certainly not easy to answer: how much would the CDF and D0 sensitivity curves would move toward 1 with twice the luminosity they have today? And with three times? Taking into account that to improve the sensitivity by a factor $N$ one needs $N^2$ the luminosity, they certainly still need quite a lot of additional data. And even if they claim they can improve the analyzes further more, and maybe include some other remote channel they might still miss in the combination, statistics will still play the dominant role. But if I were them, I would certainly try to keep on running anyway as long as I can.

Pessimist In Elafonisos

2010-07-24T11:54:00.005+02:00

While a thousand physicists gather in hot Paris and listen to talk after talk, I am confined in a small island of the Mediterranean, trying to relax and gather my ideas for the next few aggressive months of data analysis, a course of subnuclear physics in the fall, and of course, more reckless rumor-mongering!

Being away from the scene naturally makes me a less-well informed subject as a blogger writing for this site, but it also yields some advantages. No, I am not talking of the blue waters of Elafonisos (see picture), of the fish dinners on tables planted on the sand of a beach, or of the siestas "Greek style" which lasts from about three in the afternoon to about 6.30 in the evening.

I am talking instead of the advantage of having time to think, for once. And to have a global look at where high-energy physics is going in the next decade. The crazy schedule that experiments have forced upon themselves in order to produce as many high-quality results as possible in time for ICHEP left all of us little time to think during the last few months, and those in Paris are still immersed in the same turmoil. Let us instead take a step back and observe HEP from a distance.

After the completion of the picture of elementary fermions with the discovery of the top quark and the verification of the existence of the tau neutrino, many of us had great hopes for the future. The Higgs boson was the next target, sure, but most of my colleagues felt that besides that particle fantastic new riches were going to be at arm's reach very quickly, with the soon-to-come Run II of the Tevatron and the LHC coming thereafter.

The line of reasoning is simple: the standard model is an effective theory -of this everybody is certain. It only can be valid at the energy regime at which we are testing particle physics today; it cannot be valid at much higher energies. It does not include gravity (and we all think a honest-to-God theory of everything should do that), it does not "explain" things as we see them, but just allows us to calculate reactions and rates.

Our deeply-rooted Illuminism demands that there be something else in store for us, an apocalypse (I am studying Greek, and so I was reminded recently that "apocalypse" means "revelation")! A revelation of why the masses of fermions are so different from one another, why electroweak symmetry is broken, why neutrinos mix and what causes them to do that. And the place where this something else should reveal itself is... around the corner! It was around the corner ten years ago, it surely is there now!

There was a hint that we did get, just before the last ten years of draught. The observation that neutrinos mixed among themselves was taken as an appetizer of a twelve-entrees dinner to come. Was it true ? So far, not true. After the appetizer, we have been starving. The Tevatron experiments, Belle, Babar, and a host of other apparata have provided a wealth of new knowledge about the inner workings of the standard model, the details of quantum chromodynamics, etcetera. But nothing was found beyond the picture we had drawn, and which we already knew all too well. We are as much in the dark about what exists outside as we were ten years ago. One is reminded of that old sentence, "Consistency requires to be as ignorant as you were a year ago"...

Is it going to be supersymmetry ? Or new generations of matter ? New vector bosons, gravitons, extra dimensions ? Or still more unexpected things ? For many, the hope of a new golden age of particle physics similar to that of the late fifties of last century seemed almost a certainty ten years ago. Is it right to nourish the same enthusiasm today ? Is insisting with that enthusiasm the right thing to do with the large number of students pressing from below to do fundamental physics today ?

We keep repeating to ourselves that the LHC is a discovery machine, and that if something is there, ATLAS and CMS will find it. I cannot object to this reasoning -the LHC is a unbelievable project, the most technologically complex endeavour that humanity has produced. But I do not see why new physics had to hide in the corner of phase space where the Tevatron and the other machines could not reach it yet. The LHC is great, but this does not prove it will discover anything: it is simply a non sequitur!

These ten years have seen us exploring large chunks of the parameter space of dozens of new physics models, in vain. I may be a pessimist, but I have the feeling that the LHC will only repeat the play, at a bigger scale.

I have bet 1000 dollars four years ago that we would not see any hint of Supersymmetry at the LHC. The original bet was targeting 2010 and 10 inverse femtobarns of collisions as the year and conditions for a payoff, one way or the other. Now with the delays of the machine, we are still forced to wait. According to tentative schedules of running and shutdowns at CERN, the 10 inverse femtobarns of analyzed data that we took as the basis of a conclusive deadline to assess the bet are not going to be collected before 2013. But the bet still stands, and alas, I am as convinced as I was four years ago that the next ten years will be the apocalypse of our short-sightedness.

Will I be happy if I eventually cash my bet ? Of course not! If you know the gambling game of Blackjack, you understand that the 1000 bucks were placed as an "insurance bet". But I do not see a ten coming after the ace.

Day two -- the lattice track

2010-07-24T11:46:00.003+02:00

Friday was the day of the lattice QCD track, which I attended exclusively throughout the day. The talks in the lattice session had actually been selected to be accessible and of interest also to people outside the lattice community (in particular there were a number of review talks), so it was a bit of a pity that the talks were attended almost exclusively by lattice theorists.

Stefan Schaefer started the first session with a review talk about pion physics and progress in simulation algorithms in lattice QCD. The low-energy physics of pions is described by Chiral Perturbation Theory (χPT), which is an effective field theory with a number of "low-energy constants" that need to be determined from the underlying ("high-energy") theory, i.e. from QCD. In order to determine these constants from lattice simulations, one needs to be able to simulate at low pion masses, so that one can establish the link to the domain of validity of χPT. This is where algorithmis developments come in, since ten years ago simulations at those quark masses would have been considered impossible, even given today's computers. The reason is that the effort needed for a lattice simulation grows rapidly with decreasing quark masses; in order to curtail that growth, new algorithms were needed. One such algorithm is the domain-decomposed Hybrid Monte Carlo (DD-HMC) algorithm by Martin Lüscher, other choices include the Rational Hybrid Monte Carlo (RHMC) algorithm. Further improvements have been obtained using "deflated" solvers for the Dirac equation, which separate the low-lying (infrared) modes from the ultraviolet, and thus manage to speed up calculations considerably.

Next was Takashi Kaneko speaking about disconnected quark loops. Many quantities are difficult to measure on the lattice, because they involve not only quark flows going from source to sink, but also quark loops attached only to one side, so-called disconnected diagrams. These are very hard to measure, since they require an "all-to-all" quark propagator, i.e. in principle the complete inverse of the Dirac operator on a given gauge background. A number of techniques have been developped to deal with this problem, all of which revolve around the idea to estimate the inverse of the Dirac operator stochastically, with a variety of methods devised to beat down the resulting noise in order not to lose the signal completely.

The last talk of the session was Dru Renner talking about the calculation of the leading-order hadronic contribution to the anomalous magnetic moment of the muon from lattice QCD. The anomalous magnetic moment of the muon has a well-known and persistent 3σ discrepancy between theory and experiment; however, the theory error is dominated by the error on the hadronic contributions, which currently are being estimated phenomenologically from the cross-section e⁺e^- --> hadrons. A lattice calculation would provide a first-principles predictions for the hadronic contribution, and the leading hadronic contribution is given by the photon self-energy insertion, which can be measured as a current-current correlator on the lattice. Low-momentum contributions are dominant, however, and on a finite lattice there is a lower bound on momenta, so that the accuracy of the lattice prediction is still limited at present. Nevertheless, its good agreement with the phenomenological number provides a valuable cross-check. Determining the next-to-leading hadronic contribution will involve measuring light-by-light scattering amplitudes on the lattice, which is a completely unsolved problem as yet.

The second session had Owe Philipsen speaking on the QCD phase diagram at finite baryon desnity. At finite baryon density, i.e. finite baryon chemical potential, the fermionic determinant is in general no longer real and positive, and thus can no longer be interpreted as a probability measure, which prevents direct lattice simulations. A number of methods to work around this problem have been invented: one can Taylor-expand in the chemical potential and measure the Taylor coefficients at zero chemical potential, or one can simulate at a number of imaginary values of the chemical potential (where the determinant is positive) and analytically extend a fit to real values, or one can try to use reweighting techniques. Using a variety of these techniques, a number of groups are now exploring the QCD phase diagram in the region of low chemical potential, which however will probably be enough to clarify the existence, nature and location of the QCD critical point in the near future.

George Fleming gave a talk on the subject of "walking" technicolor models and lattice simulations of gauge theories that are not QCD-like. Depending on what the LHC finds, this may become a very very hot topic in the near future, as theorists may need to find a description of possible strong dynamics that exist at the TeV scale and give rise to electroweak symmetry breaking instead of a Standard Model Higgs.

Also related to Beyond-the-Standard-Model (BSM) physics was Philipp Gerhold's talk on the effects of a possible fourth-generation on Higgs boson mass bounds. By simulating a Higgs-Yukawa model (i.e. the Higgs and fermions, but no gauge fields), he has derived upper and lower mass bounds on the Higgs in the presence of a fourth generation; the result is that if a fourth generation with a top'-mass of about 700 GeV exists, then the Higgs mass is bounded to lie between about 600 and 900 GeV, or in reverse that if the Higgs is significantly lighter than that, there cannot be a fourth generation coupling to the Higgs in the same way as the first three generations.

For lunch, I met with some of the other ICHEP bloggers, which was very nice.

In the afternoon, the first session was started by Sinya Aoki with a talk about determining nuclear potentials from QCD, which is the programme of the HAL ("Hadrons to Atomic nuclei from the Lattice") collaboration. The potentials obtained from measuring Bethe-Salpeter amplitudes on the lattice and determining the non-local potential from them and expanding it in a derivative expansion (assuming an energy-independent potential) agree quite well with the qualitative features of phenomenological models used so far.

Colin Morningstar spoke about the calculation of the excited state spectrum using the "noisy LapH" or "distillation" method, which is a new method for improving the signal-to-noise ratio in lattice measurements of mass spectra. The method is based on realising a smearing operator for the quark fields as a projection on the space spanned by the lowest few eigenmodes of the 3d Laplace operator (hence the name Laplace-Heaviside or LapH), and on introducing a stochastic estimator of the inverse of the Dirac operator from this subspace.

This was followed by a presentation on nucleon structure calculations results from ETMC by Constantia Alexandrou. Nucleon structure is quite hard on the lattice -- there are significant systematic effects from the unphysically heavy light quarks used in simulations, and disconnected diagrams also contribute in many cases. As a result, lattice predictions disagree with experiment in many cases, although there is a notable tendency towards the experimental data as more sources of error come under control.

Jose Rodriguez-Quintero gave a talk on determinations of the strong coupling α_s. This is a field in which a number of different methods have been used: the Schrödinger functional method pioneered by the ALPHA collaboration, calculations of Wilson loops within lattice perturbation theory as performed by the HPQCD collaboration, the use of moments of current-current correlators matched to continuum perturbation theory (also pursued by the HPQCD collaboration), and methods based on the determination of Landau-gauge propagators as presented by Jose.

The last session of the lattice track started off with Enno Scholz talking about results from 2+1 flavour QCD with domain wall fermions. Domain wall fermions are a formulation of fermions on the lattice that manages to realise chiral symmetry and evade the Nielsen-Ninomiya theorem by introducing a fictitious fifth coordinate in which the fermions are localised on a four-dimensional domain wall. The fifth dimension means that simulations are much more expensive than for other (e.g. Wilson or staggered) fermions that do not maintain the continuum chiral symmetry.

Michele Della Morte gave a review of heavy flavour physics, focussing on work done by the ALPHA collaboration. One important point he made was that in order for heavy flavours to be reliably simulated, fine lattice are needed (the Symanzik effective theory is an asymptotic expansion, and it only works for ma small enough) -- e.g. a<0.08 fm for charm quarks. For bottom quarks, this means that one needs to resort to effective theories such as HQET, which has been studied non-perturbatively by the ALPHA collaboration in the quenched case, with results for N_f=2 forthcoming.

Another review of heavy flavour physics, this time focussing on work done by the HPQCD and FNAL/MILC collaborations, was given by Elvira Gamiz. A lot of work has done into reducing the errors on f_{D_s}, and the previously reported tension between theory and experiment has vanished. Significant progress is also being made in the determination of semileptonic form factors.

The only thing I wasn't too happy with about the lattice track was that almost no non-lattice people attended, since the talks were selected specifically to give an overview of recent progress for the benefit of a wider audience (most of the lattice folks have been to the LATTICE 2010 conference and already know a lot of these results). So why don't non-lattice people come to lattice talks -- is it that the lattice has a reputation for being highly technical?

Heavy Long-lived Particles at the LHC

2010-07-24T09:46:00.010+02:00

Yesterday at ICHEP CMS and ATLAS presented their searches of long-lived charged particles. Most of the particles we deal with at the LHC decay almost immediately, basically right at the collision point, and one needs to reconstruct them from their decay product such as electrons, photons, muons or jets. However, it's conceivable that there exist new particles whose lifetime is 100 nanoseconds or more, in which case they traverse the entire detector before decaying (recall that the speed of light is one foot per nanosecond ;-). If these particles have an electric or color charge, they will leave a mark of their passage. Long-lived charged particles arise in many extensions of the standard model. The best known example is gauge mediated supersymmetry with a high scale of supersymmery breaking. In this case the lightest (and stable) supersymmetric particle is typically the gravitino who itself is uncharged. But the next-to-lightest supersymmetric particle may be charged (it could be stau, for example) and couples very weakly to the gravitino, in which case it lives very long for particle physics (or even human) standards. Supersymmetry is just one example; there are tons of less or more motivated models that predict long-lived particles.

So what does such a long lived particle looks like? It slashes through the detector all the way to the muon chambers, leaving an energetic track in the tracker but little energy deposition in the calorimeters. In other words, it looks much like a muon, and in most case it will be identified as one by online systems. But it can be distinguished from a muon by looking more closely at its traces. First of all, we expect it to be heavy (100 GeV or more, otherwise LEP would have seen it before), and therefore the track will have a large transverse momentum. Another consequence of the large mass is that the majority of these particels are produced with small velocities (where small here means a fraction of the speed of light, say 0.3 -0.7c). The Bethe-Bloch equation tells us that slow moving particles will lose energy faster when passing through matter. Thus, one can find heavy long-lived charged particles by looking for an excess of tracks with high pT and high dE/dx (one can also look at the time-of-flight, or the number of hits in the tracker). CMS, who seems more advanced in this search than ATLAS, already presented the first limits based on 200 nb-1 of data. The limits are not competitive with those of the Tevatron yet, but this will change soon.
Actually, long-lived charged particle may lose so much energy when passing through the detector that they stop altogether, and then decay after a while.This can readily happen for strongly interacting particles, such as long-lived gluinos that arise for example in split supersymmetry. Since in this case the particle does not reach the muon system, the search is more challenging and also more spectacular. Namely, one searches for localized energy bursts in the calorimeter during the time when there is no collisions, e.g. between beam crossings or when the accelerator is switched off altogether. CMS has already first limits on these events, and in this case they are already better than the Tevatron limits in a part of the parameter space.
It's reassuring that from day one the LHC plunges into out-of-the-box searches that some call exotic. My hunch is that, if there's any new physics at the TeV scale at all, it will take some unexpected form that will require non-standard techniques to discover. And the added value is that in these less explored corners of particle phenomenology interesting results and non-trivial limits can be obtained relatively fast, even during the first year of the LHC running.

Futuristic

2010-07-24T09:40:00.005+02:00

Today's the day for future planners (as in: those who plan the future, not those who plan to plan in the future). I'll spend the best part of the day in Salle 251, in the session "Future Machines and Projects", and am currently writing from "Advances in Instrumentation", which also features future detectors. I just witnessed one of those moments that makes these conferences so special: the speaker was just explaining the concept of particle flow when a phone started ringing in the room. Embarrassed fumbling in various bags ensued, but the phone kept ringing. And ringing. Until the speaker said: "Never mind, it's mine..." and just continued showing his slides. (It has just rung again, he mustbe popular)
I am impressed with the discipline here. Only a few people have their laptops out on their laps, and I am the only one hacking away at it (sorry to my fellow session listeners!). The rest is listening, not just politely, but attentively. I think I can safely say 'no' to my initial question whether physicists are non-committal. After all some of the sessions are so packed that people have to stand!
Physicists also share a similar sense of humour. After the initial shock - Wha-? The President? - I overheard several people mumbling to each other "Do you think Carla will come as well?" To find out you'll have to be at the Monday morning session, properly badged and identified....

Presidential honours

2010-07-23T16:14:00.005+02:00

Since I see this on the ICHEP front page now, and on the projection screen in the auditorium, it is no longer a secret -- M. le Président de la République Nicolas Sarkozy will address the conference on Monday! It's nice to see that some politicians want to be seen in public with scientists rather than football players.

Participants need to remember to bring their ICHEP nametags and an official photo ID to the Palais des Congrès on Monday, or they won't be able to get in.

From LHC sessions: First top candidates, and one experiment has had enough!

2010-07-23T11:49:00.000+02:00

I've spent this morning in the "Early Experience and Results from LHC" sessions. They've been almost as packed as yesterday's Higgs session, and have included two interesting firsts (for me, at least).

From ATLAS, in the presentation of the first results on QCD, Quarkonia and Heavy-Flavour Physics, one of the experiment's very first top quark candidate events. I expect that many more such events are being presented by ATLAS and CMS at this very moment in the parallel session devoted to top quarks.

And from LHCf, I learned that this small experiment is the first at LHC to have finished data taking. That's right, while the big experiments are just barely getting started, the smallest experiment has already collected all the data they need and even removed their detectors from the beamline on July 20.

The LHCf speaker was quite good - unlike the big experiments, the speakers from LHCf and TOTEM don't assume that everyone in the room already knows the details of their experiment, so they spend time at the beginning of the talk explaining their physics goals and how the experiment is set up and works. As a non-specialist, this is something I really appreciate. So I learned that the goal of LHCf is to provide input to the study of ultra-high-energy cosmic rays (UHECR) created by high-energy particles colliding with Earth's atmosphere, by studying the production of similarly ultra-high-energy showers that are man-made at the LHC. I also learned that they can distinguish between the various theories that currently exist to describe such UHECRs with a relatively small amount of data from LHC collisions (compared to the ATLAS experiment, for example, around which their detector is placed).

This isn't the end of the story for LHCf; they will be back at the start of the LHC's run with 7 TeV beams, currently scheduled to start in 2013. Between now and then they will continue and refine their analysis of 450 GeV and 3.5 TeV data, and upgrade their detector to withstand the higher radiation that will come with 7 TeV beams.

Gadgets!

2010-07-23T11:47:00.009+02:00

There must be some secret competition among the organizers of different conferences, aiming to to invent the most fancy gadget to offer the participants. While probably the organizers sincerely put some effort in this conference gadget business, reality is that the average participant rarely uses them, and often tosses them to dust in a remote angle once is back from the conference.

For instance: why do we get a backpack of messenger bag with the conference logo at every single conference? I mean, nowadays everyone goes to conference with her laptop bag: why should we be always came back home with two of them? And to be fair, the quality of the conference backpacks usually ranges from horrible to barely acceptable: these bags usually fall apart after a couple of months use. Conference organizers, couldn't you spare us? Please!

For the record, this is the content of my brand new ICHEP backpack I got yesterday:

The backpack itself! I suspect the average organizer believes that the quality of the conference is directly proportional to the number of pockets the conference bag. The ICHEP backpack has even the never-used-by-anyone cell phone pocket on the backpack strap: if my theory is correct, the conference will be a huge success.
The conference badge. How many of them do you have at home? Do you keep them or do you trow them away? But beware, the ICHEP one might become very important in the next days (but I cannot yest tell you why)!
The ICHEP fancy gadget: this year we got a pen that is also a laser pointer and a 2 Gb USB drive, with the conference program and booklet already loaded on. This could probably sound great, if only the gadget pen was not so over-sized to be difficult to use as... a pen.
Paper, paper, paper. Including: the conference program, the list of participants, a issue of the CNRS magazine, a few fliers from the conference sponsors, a map of Paris, a map of the Musee de la Science, the tickets for the various social events, and even a booklet with Summer events in Paris this year.
A weird mouse-pad (or is it a magnetic badge?) with the periodic table of elements. Who ordered that?
A 10%-discount coupon for the Galerie Lafayette, that - as someone put it yesterday during the coffee break - is possibly the best gadget we got! :-)

Upsilons Popping Up In CMS!

2010-07-23T11:13:00.001+02:00

The CMS collaboration at the LHC collider has just produced its very first results on the production of Upsilon particles, with 280 inverse nanobarns of proton-proton collisions at 7 TeV center-of-mass energy. I wish to discuss these results here, to explain what is interesting in these very early measurements, and what we can expect to learn in the future from them.

The production of resonances decaying to muon pairs is one of the first things one wants to study when a hadron collider starts operation. This is because these particles are extremely well known, so one immediately figures out whether the detector is working properly, what is the resolution on the momenta of the reconstructed particles, etcetera.

Why muon pairs ? Because at a hadron collider muons are well measured and identified with small backgrounds even when they have relatively low momenta. There is a whole family of bodies which decay to muon pairs, which include the J/Psi and Psi(2S) states (bound states of a charm-anticharm quark), the Upsilon 1S,2S,and 3S family (bound states of a bottom-antibottom quark), and of course, the Z boson.

Upsilons are a real treat to see: CMS made a strong effort to analyze as much data as possible before ICHEP, to present a sizable signal of these decays. Look at the mass distribution below: it shows the three resonances standing above small backgrounds.

This signal is not qualitatively different from the one obtained with larger statistics by the CDF experiment, which has an excellent resolution for central muons: this tells us that things are in good order. The peaks of the three peaks are exactly where they should be, and their width demonstrates an excellent momentum resolution. Studies performed with this signal together with those of J/Psi mesons and Z bosons already allowed CMS to determine the tracking momentum scale and resolution with great accuracy.

However, with Upsilon mesons things are trickier, and possibly more interesting, than with the other resonances. These particles are indeed well-known, but their production mechanisms at hadron machines is actually still hiding small secrets. In particular, two things require to be stressed.

1) the polarization of Upsilon particles created in the LHC collisions is not known! This actually creates some uncertainty in the number of dimuon decays that can be collected, as a function of the parent body's transverse momentum.

Polarization is a measure of the alignment of the Upsilon spin with its direction of motion. Depending on the particle spin, the two muons get emitted in different angular configurations. This may affect their detection, particularly in a region when the Upsilon has a momentum of about half its rest mass. Have a look at the graph below, which shows the detection efficiency as a function of Upsilon transverse momentum and rapidity.

This graph was produced by assuming that the Upsilon has no net polarization (that means that no preferred configuration of the spin is implied). You can see that there is a loss of efficiency for high-rapidity (the upper blue band), where the chance that a muon misses the muon detection system increases; more interesting is to note that there is a whole band of low efficiency for transverse momenta of about 5 GeV.

What is happening is that if the Upsilon is moving at 5 GeV of momentum and it decays with one muon backward-going, the kick from the decay will cancel part of the motion of the system, and the muon will remain with insufficient momentum to reach the detection system (muons with momenta below a few GeV cannot reach the muon chambers of CMS, which are located externally of a strong axial magnetic field and a thick calorimeter).

The "inefficiency band" behaves differently depending on the emission angle of the two muons. So by not knowing the exact production mechanism, and thus not knowing the polarization of the particles, we have uncertainties in the detection efficiency. This will be turned to our advantage once we know our detection efficiency from other sources: we will then be able to determine the polarization by counting these particles as a function of momentum!

2) Upsilon particles are a good means of studying parton distribution functions. This involves comparing their production rapidity with production models which employ different parametrizations of the probability to find partons of given momentum fraction inside the colliding protons.

There are large uncertainties in our knowledge and modeling of the distribution of quarks and gluons of very small momentum inside the proton. These are, however, very important at the LHC. So we need to study light particles going forward! This is because a light particle such as the Upsilon, created in a proton-proton collision in the forward or backward direction, implies that a medium-momentum quark hit a very small-momentum antiquark. By studying the rate of production, we get useful information on the population of these small-momentum partons.

The graph below shows that already with the small statistics collected in the first few months of 7 TeV running of the LHC, we can study the rapidity distribution of Upsilon mesons. This distribution, compared with simulations, will one day allow precise tests of the parton distribution functions.

In summary, it is exciting to see known particles being detected at the LHC, and mined for information that will further our understanding of subnuclear physics. Of course, the LHC is a discovery machine, so eventually the information on the detailed working of the detectors and the production mechanisms for known signals will be turned to the advantage of searches for new physics processes... Stay tuned for these developments!

DISCLAIMER: The opinions expressed in this article are those of the author, and they do not reflect in any way those of the institutions to which he is affiliated. These include the CDF and CMS collaborations, as well as the Italian Institute of Nuclear Physics.

Day one -- afternoon

2010-07-23T10:32:00.000+02:00

I haven't been able to live-blog yesterday's afternoon session, so here there are only the parts of the heavy quark sessions that stuck with me for some reason or another:

CDF has observed two new decays of the B_s meson that are both Cabbibo and colour suppressed, namely B_s --> J/ψ K^* and B_s --> J/ψ K_S, with a significance of over 7σ.

BELLE has observed charmless B_s decays B_s --> K⁺K-, but no B_s --> Kπ or B_s --> ππ decays. They have also observed B_s --> D_s^*π, B_s --> D_s^*ρ, and B_s --> D_sρ decays.

Nicolas Garron spoke about the determination of the bottom quark mass and the B_s decay constant in nonperturbative Heavy Quark Effective Theory (HQET) on the lattice. This approach is based on a fully nonperturbative lattice formulation of HQET, the effective theory obtained by expanding QCD in powers of the inverse heavy quark mass. Using this formulation, one can nonperturbatively match HQET to QCD on a very fine (and hence necessarily very small) lattice and thus determine the HQET couplings, which then can be extended to larger lattices and be used to make predictions. The results presented were still for quenched (i.e. N_f=0) QCD, but the calculation of N_f=2 results is far advanced.

The poster session took place in the evening. Since I had a poster to present myself, I didn't really get to discuss any posters with their presenters. At the reception afterwards, the food was very nice, but was hard to obtain because the density of physicists approached a singularity near the plates.

Limits on interaction strength

2010-07-23T08:59:00.004+02:00

Advancements in our understanding of quantum field theory (QFT) are rare these days. Papa Weinberg and others worked out all the easy stuff back in the 70s, and since that time we had only isolated flashes of genius such as Seiberg-Witten or AdS/CFT. Yesterday at ICHEP I heard the talk of Slava Rychkov about one of the two most interesting advancements that happened last year.

In the real world there seems to be some constraint on the interaction strength. In QFT, the interaction strength is represented by a coupling constant which is related to the probability of a particle splitting into 2 or more particles. In the first approximation, the larger the coupling constant, the larger the interaction strength. However in all examples we are aware of the interaction strength cannot be arbitrarily increased. When the coupling constant reaches the value of order 4 $\pi$, rather than interactions becoming stronger and stronger, the theory undergoes a phase transition. A new theoretical theoretical description with new effective particles emerges, and these new particles interact with a finite strength. The well know example is QCD: at low energies when the strong coupling constant grows very large the theory of quarks and gluons rearranges into the theory of mesons and baryons who interact with the final strength.

So, is there some hard-wired limit on the interaction strength in QFT? For the moment, such limits can be derived only in the context of conformal QFTs (abbreviated as CFTs). CFTs do not describe the real worlds particle physics, as conformal invariance is not compatible with massive particles. But CFT may be an effective description of some subsystems of the real world. Condensed matter physicists in their laboratories routinely produce materials that can be well described by CFT. In our field, there are ideas that the Higgs boson could emerge from another sector that is approximately conformal sector over a large range of energy scales. More recently, particle phenomenologists entertained the idea of unparticles, that is a conformal sector that weakly couples to standard model particles so that this weird stuff can be produced in colliders.

It turns that in CFT there are concrete limits on the interaction strength, more precisely on the 3-point function of primary operators. These limits can be derived using operator product expansion (in this context, conformal block decomposition) and requiring that it satisfies crossing symmetry. The rest is some algebra and a clever rearrangement of terms in the expansion. The result as a function of the operator dimension is shown on the plot below.
So, indeed, the interaction strength cannot be arbitrarily large. This result limits the options of what can be observed at the LHC in certain unparticle scenarios. Similar methods can be used to derive bounds on possible operator dimensions in CFT.

See the slides or the original paper.

Highlights of my Day One at ICHEP

2010-07-23T00:17:00.011+02:00

With great excitements I arrived at Palais des Congres this morning for Day One at ICHEP. Most of the talks today were outside of my area of research. This is a good thing in a sense as it means that I may get to learn new things from the talks. And yes, it was indeed the case. On the experimental side, I have enjoyed the well attended Tevatron Higgs session this afternoon on updated limits in both the high and low mass regions from CDF and D0. On the theory side, the highlight was the talk on Emergent Gravity by Erik Verlinde based on the paper (arXiv:1001.0785) he wrote earlier this year claiming that

"Gravity is not a fundamental force"

but it emerges as an entropic force caused by changed in the information associated with the positions of material bodies. Given that this claim changes fundamentally how we think about gravity, there were many very critical questions from the audience as you can imagine, such as the scale at which the entropic interpretation breaks down, the role of graviton in this scenario, the equivalence of the entropic scenario and other known theories in which gravity is a fundamental force.

As pointed out by my fellow bloggers as well that the most important thing being at conferences probably is to interact with other physicists face to face. Besides the coffee breaks, poster sessions also provide such an opportunity to informally discuss and learn new results and ideas from one another.

So what did I learn during the poster session today??

"Is there LSND anomaly?"

According to Poster # 588 from Harp - CDP, which presents the new results they just released a few weeks ago on the low energy pion backgrounds, the answer is NO. What they found is that there is a significant asymmetry in producing pi+ and pi-. The effects of this was not taken into account by LSND, according to the poster. More specifically, LSND underestimated the backgrounds by a factor of 2 and it underestimated the systematic errors by at least by a factor of 2, according to Poster #588. If the analysis stands, the still unanswered question then is, could the excess seen by MiniBoone in both the neutrino and antineutrino modes similarly due to the low energy pion backgrounds?

Tomorrow will be an exciting day as well. It will start out with presentations on first physics results from the LHC. Very much looking forward to it!

Day 1: ATLAS and CMS electrons and photons, and some Tevatron Higgs searches

2010-07-22T23:55:00.013+02:00

I mostly kept my program.

Mostly, because I arrived slightly late at the Palais des Congres for the afternoon session (the French waitress at the creperie just in front of the Saint Paul church was so slow...): Salle 252B was already so packed I was not able to enter, and I decided to head directly to Salle Maillot for the LHC-calorimetry-electron-and-photon session. Pity, I missed Erik Verlinde's talk. Someone told me during the reception that apparently his presentation was followed by quite a discussion: is there any theorist out there that attended the session and wants to report?

ATLAS and CMS calorimetry, electrons and photons

In a nutshell: both the ATLAS and CMS electromagnetic calorimeters are doing rather well, thanks for asking :-) More seriously: both the experiments seem to have successfully used a fair amount of integrated luminosity at 7 TeV to commission their electron and photon triggers, reconstruction algorithms and selection procedures. And, even if probably neither of them would ever put it in this way, both the experiments still sees some small discrepancy between data and MonteCarlo in some of the variable used for the particle identification. I would dare to say that the main difference between the two is in the way the ATLAS and CMS speakers decided to address this point. The discriminating variable used to select electrons and photons don't exactly look the same in data and MC? When asked, the ATLAS speaker simply acknowledged the mismatches, and minimized saying that there's still some "work in progress" needed to understand and solve the issues. With similar material, the CMS speaker simply flashed through the plots claiming fair-to-reasonable-to-excellent agreement for all of them. By looking in more details the graphs at the end of the session (and peering to some of the posters related to the same subject), I must admit that - despite the impression someone could have had at the end of the session - I have some hard time to conclude that one experiment is doing better that the other. I'm tempted to say that the only real difference - but I'm of course exaggerating and kidding - is their public relation strategy :-) In the next days we'll see more physics-oriented results related to electrons and photons: I'm curious to see if things will get more explicitly different.

P.S. the ATLAS $J/\psi$ peak obtained using the tracks' $p_T$ is finally taking advantage of the bremsstrahlung recovery fit that was missing at pLHC, and a fair comparison with CMS can now be done.

Low and high mass Higgs searches at Tevatron

After the coffee break and the talk on Material mapping in ATLAS, I tried to enter again Salle 252B for the first Higgs at Tevatron session. Not an easy task: probably thanks to the rumor spread in last weeks, everyone was eager to see if there was really something to get excited about. Just to give you an idea, I finally managed to enter, but I had to sit on the floor for most of the session...

Not all of the searches at low Higgs mass were presented today, and the speakers were so kind to remind us at every single talk - in a perfectly orchestrated PR action - that the CDF combination and D0 combination will only be presented tomorrow, while for the full Tevatron combination we will have to wait until Monday at the plenary talk. I leave to you to browse the plots on the transparency, and eventually to try to do a combination by eye. I would probably bet on and larger exclusion region at high mass (easy). Someone braver - with a bit of imagination and a leap of faith - may imagine some kind of excess at low mass (but this is fiction, and I'm not good at it).

Three lines on the reception

Imagine 1088 hungry physicist packed in a small hall at the end of a long first day. Results: the waiters could not even go from the kitchen to the central bouvette with their finger-food trays, they got systematically intercepted along the way, and the trays emptied by a storm of locusts!

Hunting the Higgs in the Palais de Congres

2010-07-22T16:20:00.000+02:00

Having just arrived at ICHEP, it's clear that it isn't just the media who have Higgs fever; the particle physicists have caught it too. The first session I attended focused on the performance of the LHC experiments with first data - it was well-attended, but there were plenty of empty seats. I'm now in the 16:15 Standard Model and Electroweak Symmetry Breaking session, and it's standing room only to hear the five Higgs talks; four of which are low-mass and high-mass Higgs search results from CDF and DZero.

Of course one could argue that the LHC presentations are being held in a larger room (the Salle Maillot, where the presentations are also being webcast), but I think that's only part of the story. Wandering around during the afternoon coffee break I couldn't help but overhear several conversations with a Higgs theme, and I suspect the Higgs also had quite a lot to do with the presence of a good number of journalists at the Palais de Congres during these first parallel sessions.

Can't wait to see what the presentations will reveal, this afternoon and over the next week - and how the physicists in attendance (including my fellow bloggers) will interpret them!

Low-density physicists?

2010-07-22T11:41:00.003+02:00

One thing that struck me when arriving at the Palais des Congrès was how sparsely populated it was by ICHEP participants. We tend to think of ICHEP as a large conference with more than 1000 participants, but in many other fields that would be a small conference; when talking on the phone to a friend who is a physician and mentioning ICHEP, she said "Well, I suppose for particle physics that's a lot of people" adding that a congress of hematologists she attended had more than 10,000 delegates. I suppose that technical or political gatherings will often be even bigger, and the Palais des Congrès is clearly laid out to handle those. ICHEP is thus a low-density conference when measured on the scale of low-energy congressology; physicists being physicists, the density becomes quite high near the coffee bar during breaks, though!

Day one, session two -- Heavy Quarks

2010-07-22T11:11:00.004+02:00

The first talk of this session was given by Matthias Steinhauser, who spoke about the three-loop heavy quark potential. The heavy quark potential, which describes the forces acting between two quarks in the limit in which those quarks are taken to be infinitely heavy, is an important quantity in QCD -- its non-perturbative linear rise at large distances encodes confinement, and its perturbative short-distance behaviour gives a way to define the strong coupling α_s. It also plays a role in theoretical descriptions of quarkonium physics. The three-loop calculation presented is the result of long and very hard work by theorists.

This was followed by experimental results for radiative transitions between charmonium states, presented by Gang Li, and radiative decays of charmonia, presented by Rong-Gang Ping, both from BESIII, and on the properties of ψ resonances by Korneliy Todyshev from the KEDR expriment at the Russian VEPP-4M collider.

Moving from Charmonium to Bottomonium, but staying with experimental, Bryan Fulsom presented BaBar results for Υ(1D_J) states, finding the Υ(1D₂) at 10164.5(8)(6) MeV, for the η_b, where the Υ(1S)-η_b(1S) hyperfine splitting is about 70 MeV, a fair bit larger than theoretical predictions. It should be noted, however, that those theoretical predictions rely on effective field theories for heavy quarks, and that the next order in 1/M may account for the difference. Some indications for the h_b were presented, but the significance was below 3σ.

First day, first session -- Beyond QFT

2010-07-22T10:02:00.003+02:00

Let's try if the WLAN works -- if you can read this, it does.

The first session I attended this morning was on Theoretical Approaches beyond Quantum Field theory. The first talk there was by Gregory Korchemsky, who spoke about scattering amplitudes in maximally supersymmetric Yang-Mills Theory. Super-Yang-Mills theory is the supersymmetric version of Yang-Mills theory and its maximally supersymmetric version is the most supersymmetric theory one can have that does not include gravity. It describes a fermionic field (the gluinos, which are like quarks, only living in a different representation of the gauge group) interacting with gluons and scalar fields. The theory's symmetries are very powerful and impose tight constraints on its correlation functions -- enough to uniquely determine all tree-level amplitudes from the symmetries alone. In fact, even when considering loop contributions, some of the scattering amplitudes are determined by symmetry, while the others are at least strongly constrained.

The next talk was by Henryk Johansson, speaking about the relationship between gravity and gauge theories at the perturbative level. This approach relies on some very difficult-to-grasp hidden structures that lead to unexpected relations between tree-level amplitudes both within Yang-Mills theory and in YM theory and gravity, that can apparently be lifted to the loop level as well.

This was followed by a talk on monodromies in gauge and gravity amplitudes by Pierre Vanhove. This approach also relies on hidden structures of gauge theory and gravity, and what limited part of it I understood seemed closely related to (what limited part I understood of) the previous talk -- e.g. resolving everything into cubic vertices only.

The last talk of the session was Slava Rychkov, who spoke about constraints on Conformal Field Theories. Again using the symmetries of the theory, he derived bounds on the dimensions of operators and on the OPE coefficients or interaction strengths in CFT. This may have some applications to technicolor theories.

Theoretical high energy physics has become such a hugely diverse field that people working on different areas, even if they explore the same physics (e.g. gauge theories) using different methods (e.g. lattice simulations vs. analytical approaches), speak almost entirely different languages, and it becomes very hard to understand each other's talks; still, it is important to see what progress is being made using other approaches, even if one walks away with the humbling feeling of not understanding all that much.

Ca y est! (or something)

2010-07-22T08:51:00.002+02:00

So here we go, this is it, the conference starts! A week of results and discussions (and plans) is just starting, and it's exciting. And it's like always at big conferences like these... you meet the first colleague on the metro, the next one on the way out, and by the time you have arrived at the registration desk you have become a crowd. First confusion (where are the rooms? where are the toilets? and most importantly: what's the password for the internet connection and where is a plug for my laptop??) soon calms down, and right now people file into Salle Maillot to hear a talk about the status of the LHC (the machine, that is, rather than the detectors). Interestingly, they all stay close to the edges of the seating area, apparently ready to flee, hop into the next session, catch news about ws and zs or secure a place at the computer terminals... are physicists noncommittal? We'll see later in the week...

The Practice Talk

2010-07-21T14:53:00.001+02:00

ICHEP is tomorrow, and I write this just as I’m about to jump on a TGV to transport me from the South of France (Marseille) to Paris. For an experimentalist the last several days have been full of practice talks (and they will continue to go on past the start of ICHEP). Everyone gives them from the most senior person on down.

That might seem funny… Even senior well established people give them. The reason is pretty simple: often it saves your bacon! This is especially true for the large overview talks that many people have to give. These are long – sometimes as long as 45 minutes, and frequently cover material that the person given the talk is not expert in. See the many CDF/DZERO combined talks that will occur at ICHEP. Since you can’t be a member of both experiments there is no way one person can possibly be an expert in both experiments! The practice talk is a chance for the two experiments to sit in the same room and give the speaker advice, tell them extra information in case they get a tricky question, etc.

The other thing it does is teach. This is especially true when you are just starting out. In experimental particle physics we are constantly giving talks to our colleagues – those on ATLAS or perhaps on CMS, depending on which experiment we work. But these talks are full of jargon (“B layer”, “layer zero-zero”, etc. – who is going to guess what those are and what experiment??). Sometimes we get so used to the jargon we forget it is jargon. When I was a graduate student and beginning post-doc I had a lot of trouble with this. Practice talks were a great way to have people spot it.

Another thing is TIME. Ugh. The clock. When you are giving a talk about 50% of your concentration is focused on delivering the material, watching the audience to see if they are falling asleep, or they looked confused or puzzled. The other half is constantly checking the clock and the slide number to make sure you are still in sync. The good folks who run the session are charged with one thing above all: keep it on time. And if you go over, they can get pretty aggressive. This can be difficulty because usually the most interesting slides are the ones that come last – they contain the punch-line, the measurement, THE PLOT. So, it sucks to have to rush through that!

Finally, the old maxim: practice makes perfect. By forcing you to have a draft of the talk several days ahead of time, you have several days to do polish work. Rather than just getting the major parts of the talk up the night before you have to give it.

I remember when we started doing practice talks in DZERO – the average quality of our presentations at conferences went way up! I don’t know of any large experiment that doesn’t do this now.

How about theorists? What do they do…? :-)

ATLAS Reach Predictions for a SM Higgs

2010-07-21T11:47:00.003+02:00

The Atlas collaboration made public, just in time for the 2010 ICHEP conference in Paris, the projected reach of their searches for standard model Higgs bosons. This is a whole set of interesting new results which, although necessarily still based on simulations, tell us a lot about what we might see toward the end of next year at the LHC.

The most lucky among us will hear about these projections in the dedicated talks in Paris. I, however, have to lament my absence from the beautiful French capital: being confined in a remote island of the Aegean Sea, I can only peek at the results from a distance. In this particular case with some advance, thanks to the openness of the public ATLAS pages.

Here I will just flash a couple of the results, because the plentiful online documentation that ATLAS provided makes it a worthless exercise on my part to just echo it here. However, maybe I can comment the most relevant plots for those of you too lazy to browse the information-thick ATLAS pages.

Higgs to W boson pairs

By far the most sensitive channel for a search of an intermediate-mass Higgs boson at the LHC is the one involving the production of the Higgs followed by its decay to two W bosons of opposite charge. Backgrounds include the direct production of two W bosons without the intercession of the Higgs, as well as the production of a top-antitop quark pair, which decays into two W bosons and two additional b-quark jets.

The cleanest way to observe W bosons is to detect their decay into electron-neutrino or muon-neutrino pairs. But this unfortunately only happens two times out of nine for each W particle, because there are nine possible ways for the W to disintegrate (others being tau-neutrino pairs, plus three each of up-down and charm-strange quark pairs). All in all, if we want both W bosons to yield electrons or muons, this happens only four times in 81, which reduces significantly the yield of this already elusive process. Elusive is the word: Higgs production occurs less than once in a billion collisions!

To compensate for the loss of the hardly detectable decays of W bosons to quarks or tau leptons, we note that if the Higgs boson has a mass close to 160 GeV -i.e. twice the W boson mass- the decay H->WW is almost certain. It can still occur, at a smaller rate, if the Higgs has a mass below 160 GeV, but then one of the W bosons will be "virtual": it will be in other words unnaturally light, and its decay products will be less energetic. Because of the smaller rate and less energetic final state objects, the sensitivity of the WW channel decreases for masses below 160 GeV.

The sensitivity also decreases if the Higgs is heavier, because the heavier the particle is, the less probable is its production: this is due to the well-known feature of hadron colliders, whereby the quarks and gluons inside the colliding protons are harder to find carrying large fractions of the parent's energy, while more energetic gluon pairs are needed to produce heavier Higgs bosons.

ATLAS studied its discovery reach by first developing a very careful search strategy, and then performing it on simulated data. This allowed them to produce the information summarized in the figure below.

The figure shows the by now customary "brazil band", describing in this case what ATLAS expects to get with one inverse femtobarn of collisions, an amount of data which will be collected by the end of next year. On the horizontal axis you see the unknown value of the Higgs mass, while on the vertical axis there is the production rate of Higgs bosons, in units of the expected standard model rate. The horizontal hatched line reminds us what is the standard model expectation.

Now let us take the red points and try to understand what they mean. At 150 GeV, the red point lies at a value of about 0.8. This means that on average (but I should say "at least 50% of the time", since the points describe the median and not the mean of a distribution of limits) ATLAS expects to exclude, at 95% confidence level, that the Higgs boson has a production rate larger than twice the standard model expectation, if the particle has a mass of 150 GeV.

In other words, give ATLAS that much data, and with the WW search alone they will exclude that the standard model Higgs boson has a mass of 150 GeV. But this is only valid on average: backgrounds may fluctuate, and they may affect the limit that ATLAS can in fact set. This is described by the brazil band: all points within the green band, at the same 150 GeV abscissa, are ones that may occur 68% of the time, and all those within the green plus yellow band may occur 97% of the time. This implies that at 150 GeV the limit might well end up being at twice the standard model rate, or at 0.3 times the standard model rate: 0.8 is just the median of a wide distribution of possible outcomes of the experiment, when run on one single set of 1-femtobarn data.

Having understood what the green and yellow bands mean for a single mass point, we can see what exactly the whole curve means: while the region that on average will be excluded is, if my eyes do not fail me, 145-182 GeV (the region bracketed by the points where the red line cross the hatched horizontal line), ATLAS might be "lucky": with a 1-sigma downward fluctuation of the backgrounds the exclusion might end up being between 133 and 192 GeV (points where the lower limit of the green band meet the hatched line); with a two-sigma downward fluctuation, the limit might be all the way from 125 and over 200 GeV!

Of course, we are making two unnecessary assumptions here. The first one is that there is no Higgs boson! Of course, if the Higgs is present, the limit that will be obtained will be worse than the red line, in some region of the mass distribution. The second assumption is that the data "fluctuate" down coherently for different mass searches: you have to realize that the search details are different for different mass values, so when we say "the data fluctuates down" we are making approximations to a more complex situation.

Finally, the ATLAS note also gives the predicted minimum luminosity necessary to observe a Higgs boson in the WW final state, as a function of Higgs mass. From table 9 in their public document we thus learn that about 4.8 inverse femtobarns of proton-proton collisions will be necessary to achieve a 5-standard-deviation significance, if the Higgs mass is 160 GeV. For other masses, the required amount of data rapidly increases -but of course this is only relevant for the H->WW search alone.

Combined Reach

ATLAS produced 95% limit plots similar to the one shown above for other searches of the standard model Higgs boson; you can find all the material here. The most relevant summary is however in the combined reach plot, which takes three independent searches for the Higgs and produces a combined 95% confidence level limit on the Higgs rate.

Besides the W-pair search described above, the two others included in the combination are the search for Z-pairs, and the search for photon pairs. The former is sensitive mostly at high mass, when the Higgs may decay into two real Z bosons; the latter provides sensitivity in the low-mass region, where the decay of the Higgs into just two light quanta is at its highest rate -albeit still roughly once in a thousand times!

By now, you have all the information needed to decode the graph, so here it is below.

We learn that one inverse femtobarn of collisions will allow ATLAS to exclude, on average, quite a large chunk of Higgs boson masses! The exclusion will likey be between 135 and 190 GeV, but a "2-sigma lucky" downward fluctuation of backgrounds in the data might allow to exclude from 120 GeV all the way to 200 GeV or more.

Note that ATLAS has not included in the summary a couple of search channels that will provide added sensitivity in the low-mass region: notably, the search for decays to tau-lepton pairs and the one for b-quark pairs. If you add to this the fact that this is only half of the sky -the other half being CMS, the competing experiment at LHC, which is likely to have a similar sensitivity to the Higgs boson- you might well take home an interesting concept: if the Higgs boson does not exist, we might get a significant hint of its being a "fairy field" already toward the end of next year.

Parallel sessions

2010-07-21T10:22:00.009+02:00

ICHEP is a huge conference, and with huge I mean there will more than a thousand participants, and a loooot of presentations. The standard way to pack a lot of contributions in a huge conference is to hold parallel sessions: talks belonging to different "tracks" (read: with an affine subject) are grouped together, and get presented in different rooms at the same time. So far, so good: if you are interested in a single track - let's say, in anything related to Heavy ions collisions and soft physics at hadron colliders - life will be rather easy for you. Tomorrow you'll just look for Salle 253, sit there all day, and you'll be done. But one might have more eclectic interests, and this is where things become more complicated.

If there is something I learned the hard way about huge conferences with lots of parallel sessions, is that the speakers are strictly not allowed to speak more that the time they were allocated: the participant-with-eclectic-interests must be able to trust the schedule she has been given at the beginning of the conference, and to jump from one room to another just to hear that single presentation talk in that particular session. Chairpersons at huge conference can be very mean with talkative speakers: I wonder how the ICHEP ones have been instructed...

Anyway, that's my program for tomorrow, first day of the conference. The morning will be rather easy: I'll be traveling from Geneva to Paris and will miss all sessions, so I have no real choice to make :-) . But even if I was there, it would be easy: from 9 to 10:30 I'd go to Salle 252B for The SM and EW Symmetry Breaking session and hear about W and Z boson production at HERA, Tevatron and especially at LHC by ATLAS and CMS, and stay there for the same session after the coffee break, for some top-quark galore.

My plan for Thursday afternoon looks a bit more complicated. For hors d'œuvre I plan to go to the Beyhond Quantum Field Theory Approches session in Salle 252B, where from 14 to 14:42 (sic!) Erik Verlinde will be talking about Gravity as an emergent force. There has been some fuss about his work in the last weeks, so I'm curious to hear about it in person, and especially to see the discussion. I'm definitively not a theorist, so this will be even more fun.

From 14 to 15:45 in Salle Maillot there's the Early experience and results from LHC session: I'll skip the Particle ID at LHCb talk, run back from Salle 252B for the CMS and ATLAS calorimetry talks, and especially for the CMS and ATLAS Electron and Photon performance talks.

After the coffee break I'll jump back again in Salle 252B, where the The SM and EW Symmetry Breaking session will take place, to hear about the SM Higgs boson searches at Tevatron. But not immediately: I would probably skip the first talk (Higgs prduction at the Tevatron: theoretical prediction and uncertainties) to be again in Salle Maillot for the Material Studies with photon conversions and energy flow at the ATLAS experiment talk, again in the second part of the Early experience and results from LHC session. In the same session there will be a couple of interesting jet-related talks, but I cannot really miss the Higgs-at-Tevatron talks (even if the D0-CDF combination will not be presented before next Monday), so Salle 252B will be were I'll end my Thursday.

Now the question is: how far is Salle 252B from Salle Maillot? :-)

Time and again

2010-07-21T07:27:00.003+02:00

The day before... no, not Christmas, ICHEP. As usual just before the premiere, everybody starts being quite hectic: the speakers, the organisers, and the airports. Well, actually, not so much for the latter, since we have a strike from air controller today -- which however should remain limited (so they say on the radio this morning). Let us hope that everybody arrives safely (if late) at their hotels.

Obviously, the last days before the opening are quite tense for the organisers. There is all the preparation of the material (bags, programmes, goodies, computers, cameras...) and its installation on the venue, rather obviously. But it is not the end of the story, far from that. With the help of the conveners, you have to fix the last details of the schedule --and with three days of 6 parallel sessions running at the same time, you are bound to encounter some unexpected troubles : a thousand people starts being a large statistical sample, with rare events (cancellations, changes of speakers...) occurring more often than you would expect naively.

Last but not least : check that each presenter will upload his/her talk on time, so that each session runs smoothly and without delays... I guess that this taks will keep us quite busy during a good part of the conference, especially during the parallel sessions where the organising committee will have to be delocalised over different parts of the Palais des Congrès. It is actually a general rule : the organisers spend half of their own conference out of the rooms where physics is discussed, because they have to sort out all kinds of last-minute small issues...

But I can always day-dream and assume that everything is so perfectly organised right now that I will be able to sit down, relax and enjoy the conference as any participant will. What would I pick for the parallel sessions, in addition to the talks directly related to my own work ? Obviously, some of the LHC talks. I bet that I will get out of the room with the comforting impression that the LHC experiments have started understanding the details of their detectors and of the Standard Model physics that they will have to fight against as a background for their studies. But that should not go beyond that. I will definitely go and listen to the CDF/D0 talks, even though I had already information in smaller workshops on the flavour-related results that we discussed here some time ago. I will add theoretical works beyond the Standard Model, and a couple of other talks from experiments out of my field, just to get recent news from neutrinos (Miniboone, Minos...) or from astroparticles (Fermi...). After all, the whole point of these big-scale conferences consists in listening to talks that you would not go to in more topical conferences, devoted to one or two precise subjects !

Since I am of a rather pessimistic nature, I preferer to keep my expectations low. This is the best policy to be surprised happily by the unexpected rather than disappointed by something too much awaited. Therefore, I do not expect outstanding surprises from all these talks. But who knows ? I might easily be proven wrong here. And even if we do not get striking news from the LHC, or from other experiments, there are plenty of ways for a conference to be surprising, if not on Thursday (for the beginning of the parallel sessions), maybe on Monday (for the official opening of the conference) !

Let's the fun start !

Two Days to Go!

2010-07-20T23:24:00.003+02:00

Two days before the ICHEP meeting, and I am now sitting at LAX waiting to board my flight for Frankfurt en route to Paris. Finally! It has been really hectic for the past few weeks as the pressures of getting ready for ICHEP were building up.

Like the experimentalists, theorists also have to make the last minute fine-tuning on slides, making the plots looking pretty, you name it, albeit being an operation at a much smaller scale when compared to the experimentalists. For these, my graduate student has been working extremely hard during the past month or so with some days apparently skipping lunch and sleep, for a project that we are collaborating... This is also what another collaborator of mine and I have been doing for another project that he will be presenting at ICHEP.

Unlike the experimentalists, though, theorists do not have the luxury of generating the excitements associated with the anticipation of new experimental results. Of course everyone is eagerly awaiting to hear the status updates on LHC and on Tevatron Higgs searches. On the flavor physics front, MEG will be presenting its first summer talk on its current status on mu to e + gamma searches at ICHEP. With the anomalies in MiniBoone and Minos that were presented at Neutrino 2010, there have been several theoretical speculations on the sources of these anomalies (assuming they are not due to low statistics or low energy backgrounds). I am sure we will hear about these speculations at ICHEP too.

Now it's time for boarding. Look forward to an exciting meeting in Paris!

Sneak peak at Tevatron results for ICHEP

2010-07-19T21:39:00.004+02:00

You don't need to wait for ICHEP to see some of what the CDF znd DZero experiments at Fermilab's Tevatron will present. Fermilab's daily online newsletter reports today that "a large fraction" of the Tevatron results to be presented at ICHEP were shown this past Friday in the laboratory's "wine-and-cheese" seminar. The talks from CDF and DZero are available for online viewing.

And a note for Higgs-watchers: you'll have to wait until Monday, July 26 for new results from the Tevatron experiments' combined Higgs searches.

Expectations for ICHEP

2010-07-19T07:46:00.002+02:00

3 days before the kick off of ICHEP'10 is the high time to share my hopes, fears, and expectations. Definitely, the conference is going to be a unique experience for me because it's my first time as press :-) But from the scientific point of view my emotions are mixed. Like most fellow theorists I'm rather skeptical about supersize conferences such as ICHEP. This sort of meetings used to play an important role as the venue to learn of new ideas and results in the field. Today, however, when information travels faster than light, when almost every relevant result is instantly available via arXiv, and when one can skype anyone on the globe with just one click, big conferences seem to be a relic of the 20th century. Personally I don't expect to learn of any new theoretical results in this conference, even if listening to first-hand presentations is always illuminating and may add to my understanding.

Fortunately for ICHEP things can be interesting thanks to experimentalists who generally have a completely different view of the conference. The working cycle in experimental physics often involves finalizing results for big conferences such as ICHEP, Lepton-Photon or Moriond, so as to get more exposure. This is perfectly understandable. After spending the entire year in soggy basements where the only light is the laptop screen experimentalists should excercise some decorum when the results of their work are presented to the world. This approach guarantees a decent number of brand new experimental results for this year's ICHEP.

What am I waiting for the most? This summer is very special because the first serious LHC results will be presented. Of course, none of these results could even vaguely be interesting for a wider public. With merely a couple hundreds inverse nanobarn acquired so far, and only a fraction of that analyzed, the LHC can only access bread-and-butter physics such as W,Z, or b-quark production. Still it is interesting to watch the baby growing, even if all it can say is ba-ba-ba. The new results will give us a feel of the LHC performance, and should soon provide some indirect benefits in the form of better simulation tools for more interesting processes.

And then there is the old Tevatron who does not want to pass away just yet. The most expected results that will be presented at ICHEP are related to Higgs physics. The National Enquirer's reports that Higgs has been discovered turned out to be slightly exaggerated, so instead we are going to see new limits on the Higgs mass and cross section. The other hot issue at the Tevatron right now is flavor physics and CP violation, following D0's claim that they found evidence for new physics in Bs meson decays. ICHEP will add at least one important result: the update of the D0 measurement of the CP violation in Bs to J/Psi Phi decays. The earlier D0 measurement was 2-sigmish away from the standard model, while the latest CDF update is completely consistent with the standard model. We will see soon whether we'll have more or less reason to believe in new physics in the Bs meson system. On top of that, the Tevatron will show interesting result from top physics and from a number of standard and non-standard new physics searches. Not that there is any hope of positive signals in the latter...

What else? There seem to be interesting sessions devoted to dark matter searches, although in these cases the crucial results will probably first be shown on more specialized conferences such as TeVPA or IDM. Neutrino physics has recently become a bit more interesting due to weird hints from miniBoone and Minos, and we'll get a lot of neutrino exposure during ICHEP. Maybe something else that I haven't thought of could surprise me? As long as there's coffee, there's hope.

Rumors

2010-07-18T23:13:00.000+02:00

Who doesn’t love a little gossip every now and then? Tommaso, as is usual for his style, started one the other day – that the Tevatron claims to have seen a light standard model Higgs. For whatever the reasons (both positive and negative) this one caught like wildfire. You know its big when it shows up on this site!

But to make this really work – and I mean really work in that rumors provide some sort of science payback – you need a conversation. Someone has to say the rumor (to start it). Then someone else picks it up and perhaps spreads it. But then people want to join in on the conversation. For example - “there is no way that can be true because of x, y, and z”, and then someone else says “ah – but you forgot that x can be zero, and if that happens then y and z don’t matter, and this becomes possible once more! Ha!” And so on.

That part of the rumor and gossip conversation I do consider useful. For one thing, it makes the gossipers smarter. Assembling the set of critical arguments to work out the possibility that a rumor is true or not often takes some pretty deep knowledge of the field. And from this discussion new ideas can come as well. For example, I learned quite a bit by watching the gossip and speculation after the CDF forward muon results (shoot, I can’t find a link for that result!) and DZERO same sign asymmetric muon results were released.

Which brings me to the real reason I’m writing this result. That conversation. Currently I see it on Facebook. Seriously! In my circle of friends there are a bunch of experimentalists and theorists who get into some pretty serious discussions using nothing more than one or two lines of text – or perhaps a paragraph. It is excellent. Facebook does an excellent job of re-creating the water cooler – a bunch of people standing around. Though it is better – even if you have to be busy in a meeting or teaching a class you can skip out and rejoin the conversation and read up on what everyone said! This style of conversation is also very important for someone like me: who is constantly traveling and can’t always drop by an office to catch up on the latest.

Before Facebook this happened mostly in blog comments. It still does, and the quality is quite high there, but it seems like it moves more slowly. How about twitter? I did a search over there, and there is lots of chat. But the quality level is pretty low – it is mostly people riffing of the idea or pointers to articles, etc. Are there other places people discuss this sort of thing, but at a physics level? Or perhaps better ways to use tools like twitter to find good conversation? Where else do these conversations occur in the modern digital world?

Favorite Higgs joke seen on twitter. :-)

P.S. It is almost impossible for me to participate in this particular rumor/gossip because I am an active member of DZERO, one of the two Tevatron experiments. No matter what I say, I’m screwed. So, best not to say anything. Sorry if you came to this post expecting something else!

Luminosity is not the whole story

2010-07-17T15:07:00.020+02:00

One month and a half ago I was playing the fortune-teller, and trying to guess what ATLAS and CMS will be showing at ICHEP as a function of the integrated luminosity they would have collected. ICHEP will start in a few days: today in principle we should be able to revisit that list, and to make more-than-educated guesses on the LHC results we will see in Paris.

Let's start with the luminosity. How much of it have been delivered by the LHC and secured by the experiments? The answer to the first part of the question is on the LHC Programme Coordination web pages, from which I took this plot:

As of today, the machine gave us more than 250 nb^-1: not bad at all. But if the the LHC delivers luminosity, the experiments have to record it, and the data taking is not necessarily 100% efficient: the detectors might not be ready when the "stable beam" flag is declared, or some of their parts can malfunction during the run, maybe even forcing to stop and restart the data recording.

In this respect, the experiments did rather well: as you can appreciate from the plot above, the ATLAS data taking was globally around 94% efficient since the end of March. Again, not bad at all! I was not able to find a similar public plot for CMS, so I will naively assume a similar efficiency (anybody from CMS out there, that can point me to a public results?), or even a better one. We are then left with slightly less than 250 nb^-1: according to the list, we could then safely bet on a W and Z evidence (and possibly on a bold first cross section measurement, at least for the W), on measurements of prompt electrons, photons and maybe muons, and on a lot of jet-related items.

But luminosity is not the whole story. In order to be shown at conferences, data have to be properly understood (and this take time), and results have to be approved (and this can take even more time, as Gordon recently reminded us). For instance, a good part of the ATLAS result-approval procedure for ICHEP took place during the last ATLAS week in Copenaghen, nearly three weeks ago! Look again at the luminosity plot, and see where we stood then: the LHC decided to increase the number of protons per bunch and the number of bunches immediately after!

Ok, none of us would be so lazy not to (at least try to) update the results with 10 times or more the data. But more data means more precise results, and funnily enough more precise results means more things to be understood. The point is those discrepancies that were hiding under the statistical fluctuations a month ago are now clearly rising their head to say "Hello! Try to guess what I am and what I mean...". As usual, things are often more complicated that they seem at the beginning.

These days both ATLAS and CMS are hectically reviewing the last-minute results exploiting the largest data samples possible, and I know there will still be approval meetings on both sides next Tuesday and Wednesday (ICHEP begins next Thursday!). I am really looking forward to see what we will get!

Provocative 3-sigmas results theorem

2010-07-16T21:27:00.003+02:00

Theorem: any experiment that is scheduled to be turned off will have provocative 3-sigma results.

Lemma: any experiment that has been running a long time and measures many things will have provocative 3-sigma results.

Sean Carroll (via Twitter)

Muonic Hydrogen and Dark Forces

2010-07-16T07:46:00.001+02:00

The measurement of the Lamb shift in the muonic hydrogen has echoed on blogs and elsewhere. Briefly, an experiment at the Paul Scherrer Institute (PSI) measured the energy difference between 2S(1/2) and 2P(3/2) energy levels of an atom consisting of a muon orbiting a proton. Originally, this excercise was intended as a precise determination of the charge radius (that is the size) of the proton: in the muonic hydrogen the finite proton size effect can shift certain energy levels by order one percent, much more than in the ordinary hydrogen, while other contributions to the energy levels are quite precisely known from theory. Indeed, the PSI measurement of the proton charge radius is 10 times more precise than previous measurements based on the Lamb shift in the ordinary hydrogen and on low-energy electron-proton scattering data. Intriguingly, the new result is inconsistent with the previous average at the 5 sigma level.

As usual, when an experimental result is inconsistent with the standard model prediction the most likely explanation is an experimental error or a wrong theoretical calculation. In this particular case the previous experimental data on the proton charge radius do not seem to be rock-solid, at least to a casual observer. For example, if the charge radius is extracted from electron–proton scattering the discrepancy with the PSI measurement becomes only 3.1 sigma; the PSI paper also quotes another recent measurement that is completely consistent with their result within error bars.

In any case, whenever a discrepancy with the standard model pops up, particle theorists cannot help thinking about new physics explanations. Our folk is notorious for ambulance chasing, but actually this is one of these cases when the ambulance is coming straight at us. Recently the particle community has invested a lot of interest in studies of light, hidden particles very weakly coupled to the ordinary matter. One example is the so-called dark photon: an MeV-GeV mass particle with milli-charge couplings to electrons and muons. This idea is pretty old, but in the past 2 years the interest in dark photons was boosted because their existence could explain certain astrophysical anomalies (Pamela). The signals of dark photons and other hidden particles are now being searched for at the Tevatron, LHC, B-factories, and in dedicated experiments such as ALPS at DESY, or APEX that is just kicking off at JLAB. No signal has been found in these experiments yet, but there is still a lot of room for the dark photon as long as its coupling to electrons and muons is $\epsilon \leq 10^{-3}$ smaller than that of the ordinary photon, see the picture borrowed from this paper. The news of the muonic Lamb shift came somewhat unexpectedly...but not to everyone: here is a passage from a 2-years old paper:

For example, the dark photon contribution to the electron-proton scattering amplitude at low momenta is equivalent to the $6 \epsilon^2 /m_A^2$ correction to the proton charge radius (...) It remains to be seen whether other precision QED tests (e.g. involving muonic atoms) would be able to improve on the current constraints.

So here we are. In the coming weeks we should see whether there exist concrete models capable of fitting all data. In any case, a new front in the battle against dark forces has just been opened. Now, could someone make us a muonium?

How do you know ICHEP's just around the corner?

2010-07-15T20:17:00.001+02:00

Two ways you know that ICHEP is imminent:

The blogosphere, Twitterverse and science media are abuzz with rumors regarding possible new results;
The families of particle physicists worldwide bemoan the temporary disappearance of their loved ones.

Here at CERN, activity among scientists involved with the LHC experiment has reached fever pitch. Students, postdocs and senior physicists alike are consuming even more coffee than usual, as they work to put the final touches on data analyses, results, posters and PowerPoint presentations.

With the size of today's particle physics collaborations - peaking at more than 3,000 people each for the ATLAS and CMS collaborations - getting a result approved for presentation at a major conference like ICHEP is no simple task. With so many people involved, the collaborations put in place rules and procedures governing approval processes for results and publications, with the goal of ensuring that all results have passed rigorous review and that every collaboration member has the opportunity to review and comment on every result. For the LHC experiments, the rules that have been painstakingly assembled over a decade or more have been getting their first real workout this summer, first for the PLHC conference in Hamburg, and now for ICHEP.

To give you an idea of the work behind each plot presented in an LHC talk at ICHEP, here's a quick overview of the steps involved (symmetry Magazine has a more in-depth look at this data-to-discovery process).

A small group of a few to a few dozen physicists spends weeks, months, or years analyzing data and preparing a result. Depending on the nature of the result, it may or may not be combined with result(s) from other group(s) within the same collaboration. It's then unveiled to the entire collaboration for review, and after a certain period where any collaboration member can comment (remember: 3,000 people!), receives final approval for presentation in public. Or not, in which case the process starts all over again.

And that's all just the first step. Once the result is approved, be it in the form of a plot, chart or paper for publication, then the work begins for the people who have been selected to present it and other results in public at a conference like ICHEP. They select from the approved results, prepare their poster or PowerPoint, go through a similar approval process, and practice in front of their colleagues in the collaboration.

And in cases like the current Higgs limits from the Tevatron, results could be combined from two different experiments, which doubles the complexity of the entire process. It's no wonder that, around the time of major conferences, sleep is a rare commodity.

(But even with ICHEP just around the corner, I bet some of those same hard-working scientists will take a few hours off to attend CERN's annual music extravaganza, the Hardronic Festival.)

I'll end with a quick introduction, since I'm the new girl on the blog - I'm a nuclear physicist turned science communicator, working for Fermilab Office of Communication but based at CERN for the past three years. I spend most of my time telling the story of the LHC project and U.S. scientists' involvement to journalists and the public, and the rest of my time trying to keep up with developments in the larger world of particle physics. So I can't wait to see and hear what's presented at ICHEP, and to write about it here and at the other blog I contribute to, symmetry breaking.

The ICHEP Shuffle

2010-07-15T17:09:00.001+02:00

Or maybe it should be called the ICHEP Crunch. We are one week out now. And from an experimentalist at-a-large-collider-experiment’s point-of-view, ICHEP is almost settled. The almost-final versions of all the plots are prepared, the supporting text sanitized of jargon is being fine-tuned, and the large collaborations are getting their last review in of the results. Heck, even some early results are staring to trickle out (CDF has released a two mass Higgs searchs: WH and ZH associated production (no excess!!) and a search for a standard model higgs decaying to taus (find all CDF results here), and DZERO has a search for a t’ quark (find all DZERO results here).

It feels a little bit like the ‘night before Christmas. It is the calm that is so spooky… Which is in direct contrast, of course, to what is going on outside the experiments in the papers and other blogs…

I’m fortunate enough to be on two of the experiments, ATLAS and DZERO, and on each experiment I’m participating in two sides of the shuffle. On DZERO I have a student working on getting one of the results ready. I bet that the “ICHEP deadline” was first talked about almost 6 months ago. That is about the time that the pressure started building for the folks doing the real work in the experiments. Ever since then each time someone wants to add something new, or spend the time to better understand and reduce a systematic error, they have to ask themselves “does this mean not releasing it for ICHEP?”*

In ATLAS I help run one of the groups producing results. This is management, not physics I’m doing there. The goal for people doing this task is to make sure that all the results are high enough quality to enter the review process. Sometimes this means convincing people to drop a plot, or add an extra plot. By the time someone like me gets heavily involved in the day-to-day of the analyses being prepared we are getting very close to the internal review process.

Back on DZERO I’m also helping out with one of the reviews of an analysis. Once the analyzers have put the finishing touches on their analysis, and the people running their physics group give the ok, the analysis is handed off to an internal review group. These people are meant to be independent reviewers of the analysis, and are supposed to go over it with a fine-toothed comb. Unlike external reviewers, they have access to all the internal information of the experiment. The review is based on an “analysis note” and supporting documentation. Just the note can be 100’s of pages long in case of a complex analysis (Like the Higgs searches from CDF I mentioned above). If no problems are found this review probably takes about a week. But a fresh set of eyes always turns up new problems. It is very intense time for the reviewers and the analyzers: the reviewers send questions, and the analyzers respond as fast as they can so that the review doesn’t get stuck and miss ICHEP. BTW, if you watch the experiments you’ll often see one or two or three analyses come out just after ICHEP – these are the ones where an issue was raised and couldn’t be addressed in time to make it for the conference.

Finally, with that done, the ICHEP shuffle enters its last phase: collaboration review. The now perfected analysis along with documentation suitable for everyone to read (which may be a journal paper draft) is thrown up on an internal web page and a message is broadcast to the complete collaboration (all 600 for DZERO or 3000 for ATLAS): “Our experiment is going to release this work – this week is the last chance to raise issues before it is made public!”

At the same time this review is on going everyone is putting together their ICHEP talks, running practice talks to make sure they are high quality, and answering questions that come in from the collaboration. A popular analysis can get 100’s of comments, for example (an unpopular one might get just a few). Each comment must be reviewed and answered by the analyzers and the answers cross-checked by the internal review. At the same time public web pages are being prepared with high quality versions of all the plots and links to the supporting text and documents.

And then ICHEP starts!

So… you might ask… what defines the length of the ICHEP Shuffle? Which is about 6 months? At the two ends of the year there are two large sets of conferences. ICHEP this year is in the summer, and in the winter is the Morriond series. I’ll bet you good money that I’ll see emails with the title “Morriond Analysis Planning” hitting my Inbox the week after ICHEP is over.

But, for us in these large collider experiments, ICHEP is the breather between the dances. A chance to relax, look around, see what everyone else is doing, get some new ideas, and maybe even explore Pairs!

Prayer to the Funding Agency Reviewer

2010-07-15T12:10:00.001+02:00

Prayer to the Funding Agency Reviewer

(dedicated to those that worry about rumours)

Oh Funding Reviewer, on whose hands
Rests the destiny of full many an experiment:
Be true to yourself, and bias not
Thy sober judgement through the browsing
Of tricky sites or malicious magazines.

You were chosen, wise among the wise,
To distribute thy moneys to the worthy.
Human knowledge is at the stake:
Neglect the rumours, and listen not
To lesser souls. Let the Science be your guide.

Less of a rumour, more of a fact

2010-07-15T08:46:00.002+02:00

There's quite a debate going on in the worlds (or is it only one world?) of science, media, PR, social media, science journalism and science blogging about sources, fact checking, new ways of doing your journalistic research and new ways of communicating the results of your scientific research. The debate has been going on for a while, so maybe those ways aren't really new anymore, but things have certainly changed from the olden days. And don't we all sometimes miss the olden days? So the nostalgics amongst us, and those who believe in getting information from the source, will be pleased to hear that there will be a press conference during ICHEP where all possible results from the world of international high-energy physics will be presented.

The press conference takes place on Monday, 26 July at 1 pm, at the Palais des Congrès in Paris.

Rumors About A Light Higgs

2010-07-08T21:57:00.004+02:00

And for once, I feel totally free to speculate without the fear of being crucified. If you have followed my past blog adventures for long enough, you know that in at least a couple of occasions my posts have created some friction.

Blogging can mean walking on a rope for particle physicists involved in large collaborations - the ways of the internet are infinite, really: you never know where trouble may come from! The chance to piss someone off forces bloggers to avoid making names even when they discuss humorous incidents; the internal rules of the experiments they participate in make bloggers wary of even discussing stuff that is approved for public distribution. A daily application of self-censoring review procedures before hitting the "submit" button must be enforced.

But not this time. I am sure of one thing: I know nothing at all, so I can certainly talk about it without violating any rule! It so happens that I have heard voices about a possible new "three-sigma" Higgs effect, and I do not even know which experiment this comes from! Surely, no single experiment can get mad at me this time if I tell you what it is about, right ?

...Right. Well, I am not totally sure, but I am willing to declare that I have the right to express myself here, to some extent at least! So let me spill my guts. They are almost empty anyways...

The Rumor

It reached my ear, from two different, possibly independent sources, that an experiment at the Tevatron is about to release some evidence of a light Higgs boson signal. Some say a three-sigma effect, others do not make explicit claims but talk of a unexpected result. That the result comes from the Tevatron is for sure, since the LHC experiments do not have nearly enough data yet to search for that elusive particle, and other particle physics experiments in the world have not nearly enough energy to produce it. However, I am unable to understand whether the rumor comes from CDF or from D0.

Lest you jump at conclusions too early, I need to explain something more: despite being a CDF author, I unfortunately do not follow actively the works of the Higgs Discovery Working Group within CDF, so a Higgs excess in CDF data could well have escaped me. In principle, if I now took on digging hard enough in the internal pages of the CDF experiment I might be able to find out if this signal is coming from there, and maybe learn more about it. But there are at least a dozen analyses to dig into! Too much work - while wild speculation is more fun!

Reasoning On It

So let us take a look at the latest Higgs boson limits, released jointly by CDF and D0 last November. The dozens of analyses combined for the global limit were based on a dataset amounting to anything between 2 and 5.4 inverse femtobarns of proton-antiproton collisions, while right now the experiments have probably in their hands over 50% more processed and analyzable data.

The graph I choose to make a point is actually not one describing the limit on the Higgs boson cross section as a function of Higgs mass. Rather, let me pick the one showing, as a function of mass, a quantity that describes more clearly whether the data are background-like or signal-plus-background-like. The hatched black and red curves in the figure below show the value of the statistical estimator LLR (not going to explain you here what it is, but ask for it in the comments thread if you are interested) that the experiments would have globally observed, on average, if the higgs were there (red) or not (black). The farther the two curves are, the more sensitive the experiments are to a Higgs signal.

Also note the green and yellow bands, drawn around the expected background curve: they denote the typical extent of one- and two-sigma fluctuations expected in the data. In other words, if the Higgs is NOT at 130 GeV, say, then the LLR is expected to be on average equal to 1, but 68% of the time we may expect to find it anywhere between -1 (lower edge of the green band at 130 GeV) and +2.6 (upper edge). This is the so-called "one-sigma" band.

Now, look at the full black line. This shows the actual LLR value of the data, after the complicated analyses that sought the Higgs decay in dozens of different possible final states is processed. You notice several things.

The first thing to note is that the curve stands more in the "signal-plus-background" region for masses below 145 GeV, then going up and following the "background-only" curve for higher values.

The second thing to note is that while at 165 GeV the two LLR expectation curves are quite far apart (meaning that a Higgs boson might have produced a 3-sigma excess there, quite easily), at 120-140 GeV the curve of signal-plus-background stays on the border of the green band: the _expected sensitivity is there at most a one-sigma effect. In other words, a Higgs boson at 130 GeV would on average produce a 1-sigma deviation from the background-only curve, in the Tevatron data analyzed until November 2009. On average, though! The actual observed data, if it contained a Higgs boson, could produce larger signals, if the experiments got lucky.

The third thing to note is that the black curve in the low-mass region stands even lower than the red hatched curve! That means that the data there is definitely more signal-like than background-only-like. But is this a significant observation ? Well, no: the curve is well-contained within the yellow band. A less-than-two-sigma effect.

So, that was the situation last November. What should we expect now ? Could the black curve fall further down, hinting at a Higgs boson in the 115-140 GeV range ?

It could. In my opinion, a further fluctuation of the data, and the addition of 50% more of it, could bring the black curve out of the yellow band, toward a three-sigma signal-like effect. Is this what the rumors are about ? I do not know, but one thing is sure: we will know soon... If you are coming to Paris for ICHEP, you are among the lucky ones who will get the information first-hand from the analyzers.

An Appendix: Why Rumor Mongering ?

Why am I doing this ? I know several "serious" physicists and colleagues who have questioned this care-free attitude of mine in the past. What good does it do to shout "Higgs" every second week ?

It does a lot of good to particle physics, in my very humble, but not quite uninformed, opinion. I have made this point other times, and will not repeat it here. Suffices to say that, in a nutshell, keeping particle physics in the press with hints of possible discoveries that later die out is more important than speaking loud and clear once in ten years, when a groundbreaking discovery is actually really made, and keeping silent the rest of the time.

And there is another reason why I find this kind of rumor-mongering entertaining: maybe some informed soul out there might comment anonymously and share some more gossip about the matter with us... ;-)

Not a sound name for a physics software program

2010-06-30T12:51:00.001+02:00

Through a casual browsing of the Arxiv's hep-ph section, I got to read the following title:

"CAMORRA: A C++ Library for Recursive Computation of Particle Scattering Amplitudes"

Authors are R.Kleiss and G. van den Oord. None of which appears Italian by name, so my first reaction to the title (are these people stupid or what?) got tempered by the fact that they may just be ignorant.

Camorra is the name of one of the three main criminal organizations operating in southern Italy. From wikipedia, even a computer-illiterate could learn that

The Camorra is a mafia-like criminal organization [...] It finances itself through drug trafficking/distribution, cigarette
smuggling, people smuggling, kidnapping, blackmail, bribery,
prostitution, toxic waste disposal, construction, counterfeiting, loan
sharking, money laundering, illegal gambling, robbery, arms smuggling,
extortion, protection, political corruption, and racketeering and its
activities have led to high levels of murder in the areas in which it
operates. It is the oldest and largest criminal organization in Italy.

Giving that name to a software package is a really bad idea, like calling "scrotum" a soft ice cream, "gonorrhea" a lipstick, or "pedophile" a theme park for toddlers. But as I said, the authors are probably just unaware of the issue.

In the paper, one learns that the name "CAMORRA" comes from "CAravaglios-MORetti Recursive Algorithm". Fine, but then why not call it MOCARA or CARMORA ? None of the latter would sound offensive to the ear. Unless, of course, in southern Bali CARMORA is an association of satanists, or the like... One never knows!

I look forward to hearing what you think about this. In the meantime, I sent an email to the authors, suggesting alternative names...

On and off (stage)

2010-06-19T12:49:00.006+02:00

As you might have noticed reading the posts by Mu-Chun and Georg, summer is a the heyday season for physics conferences and workshops. This is not only because we like to gather in gorgeous places that we can enjoy where the sun shines (Athens, Sardinia... or, yes, Paris), but also for a more mundane reason : many physicists lecture at university, and summer is the only time of the year where they can leave the classroom and travel to week-long international conferences without messing up their teaching schedule -- or their colleagues'.

I like to think of conferences as theatre plays, or operas. After all, they are all cultural events, aren't they ? You have the stage (the conference rooms), the actors (the speakers, with their share of drama kings and queens, happy jesters and uninspired declamatory comedians), the spectators (the good-humoured or bad-tempered participants), the director and conductor (the lucky organisers)... and the technicians taking care of everything basically, from the curtains to the lighting, passing through the costume and the make-up (the administrative and technical staff). And quite often, feelings are quite different on which side of the curtain you stand. At best, the innocent participant can sit and relax, enjoying the happy spectacle of a smoothly-run conference. But off-stage, eveybody jumps up and down like madmen to check minute details of costumes (or microphones) or to prevent major disasters looming (like the crash of the computer network).

Inasmuch as some plays become experiences of a lifetime, and some shows remain just a pleasant evening out, a thin line exists between run-of-the-mill conferences and truly memorable ones. Part of the job of the organising committee consists in gathering the right people talking on the right subjects at the right moment, and another part to propose the right social events with the right mood... Well, you need a bit of luck too. But no matter how outstanding the speakers are, and how exciting the topics turn out to be, the organising committee must also take care of another essential aspect : infrastructure. The best play in the world needs the theater to be open and working in full order.

Murphy's law being unfortunately well obeyed in real life, bad things happen on that side, but you should solve them before they affect the participants. This is a tough lesson that I learned taking part in the organisation of conferences before the ICHEP one. My more personal version is : never organise a conference in Paris in october or november. Or you will be in the same nightmare as Mu-Chun, because autumn is the heyday season for strikes in France as much as summer is for physics conferences. Fortunately (or thoughtfully) ICHEP takes place in July... At the same time as many theater and opera festivals taking place in Southern France like Avignon or Aix-en-Provence, but I do not expect a harsh competition from that side.

Another key element that cannot go astray under any circumstance : computers. Obviously, you need a good internet connection for the participants to check out mails and papers regularly (in Paris, if not in Sardinia, as mentionned by Georg). But with more than 900 participants and almost 600 talks and posters to be presented, you need a powerful tool to manage the huge data-base of people, title, places, times... and slides required by our very dense program (we have only 3 days of parallel sessions !). And for particle physics, all this rhymes with Integrated Digital Conference, or Indico, born at CERN, like so many great computer inventions. The CERN computer team has been incredibly efficient in designing and improving the Indico system that we use, Actually, by its size and its length, ICHEP is a good benchmark to test their new version of the software, and we spent a bit of time helping them to tune the system to our own needs.

Therefore, for physics as much as for many other aspects, ICHEP as a true première... And like for all premieres, both the participants and the organisers look forward to seing the play, that is, the results that will be presented for the first time in summertime Paris !

Lattice 2010, Day Five

2010-06-19T10:08:00.000+02:00

The day started with plenary sessions again. The first plenary speaker was Chris Sachrajda on the topic of phenomenology from the lattice. Referring to the talks on heavy and light quarks, spectroscopy and hadron structure for those topics, he covered a mix of various phenomenologically interesting quantities, starting from those that have been measured to good accuracy on the lattice and progressing to those that still pose serious or perhaps even unsurmountable problems. The accurate determination of V_us/V_ud from f_K/f_π and of V_us from the K_l3 form factor f⁺(0), where both the precision and the agreement with the Standard Model are very good, clearly fell into the first category. The determination of B_K is less precise and there is a 2σ tension in the resulting value of |ε_K|. Even more challenging is the decay K --> ππ, for which however progress is being made, whereas the yet greater challenge of nonleptonic B-decays cannot be tackled with presently known methods. Chris closed his talk by reminding the audience that at another lattice conference held in Italy, namely that of 1989 (i.e. when I was just a teenager), Ken Wilson had predicted that it would take 30 years until precise results could be attained from lattice QCD, and that given that we still have nine years we are well on our way.

The next plenary talk was given by Jochen Heitger, who spoke about heavy flavours on the lattice. Flavour physics is an important ingredient in the search for new physics, because essentially all extensions to the Standard Model have some kind of flavour structure that could be used to find them from their contributions to flavour processes. On the lattice, "gold-plated" processes with no or one hadron in the final state and a well-controlled chiral behaviour play a crucial role because they can be treated accurately. Still, treating heavy quarks on the lattice is difficult, because on needs to maintain a multiscale hierarchy of 1/L << m_π << m_Q << 1/a. A variety of methods are currently in use, and Jochen nicely summarised results from most of them, including, but not limited to, the current-current correlators used by HPQCD, ETMC's interpolation of ratios between the static limit and dynamical masses, and the Fermilab approach, paying special attention to the programme of non-perturbative HQET pursued by the ALPHA collaboration.
The second plenary session started with a talk by Mike Peardon about improved design of hadron creation operators. The method in question is the "distillation" method that has been talked about a lot for about a year now. The basic insight at its root is that we generally use smeared operators to improve the signal-to-noise ratio, and that smearing tends to wipe out contributions from high-frequency modes of the Laplacian. If one then defines a novel smearing operator by projecting on the lowest few modes of the (spatial) Laplacian, this operator can be used to re-express the large traces appearing in correlation functions with smaller traces over the space spanned by the low-modes. If the smearing or "distillation" operator is D(t)=V(t)V(t)⁺, one defines the "perambulator" τ(t,t')=V(t)⁺M^-1(t,t')V(t') that takes the place of the propagator, and reduced operators Φ(t)=V(t)⁺ΓV(t), in terms of which to write the small traces. Insertions needed for three-point functions can be treated similarly by defining a generalised perambulator. Unfortunately, this method as it stands has a serious problem in that it scales very badly with the spatial volume -- the number of low-modes needed for a given accuracy scales with the volume, and so the method scales at least like the volume squared. However, this problem can be solved by using a stochastic estimator that is defined in the low-mode space, and the resulting stochastic method appears to perform much better than the usual "dilution" method.

The last speaker of the morning was Michele Pepe with a talk on string effects in Yang-Mills theory. The subject of the talk was the measurement of the width of the effective string and the observation of the decay of unstable k-strings in SU(2) gauge theory. By using a multilevel simulation technique proposed by Lüscher and Weisz, Pepe and collaborators have been able to perform these very challenging measurements. The results for the string width agree with theoretical expectations from the Nambu-Goto action, and the expected pattern of k-string decays (1 --> 0, 3/2 --> 1/2, and 2 --> 1 --> 0) could be nicely seen in the plots.

The plenary session was closed by the announcement that LATTICE 2011 will be held from 10-16th July 2011 at the Squaw Valley Resort in Lake Tahoe, California, USA.

In the afternoon there were again parallel sessions.

Hello World from Athens, Greece!

2010-06-18T14:27:00.011+02:00

Greetings from Athens where I have been for the Neutrino 2010 Conference! As this is my first post, let me just briefly introduce myself. I am a particle theorist working on beyond the standard model physics. Particularly I have been interested in the origin of fermion masses and mixing (including the neutrinos, of course!) and have been trying to gain some insights into this problem by constructing models utilizing a variety kinds of new physics (grand unification, extra dimensions, Z'...).

Getting back to the Neutrino 2010 Conference. Some of the highlights of the conference were the new results from MINOS and MiniBoone. The MINOS Collaboration updated their results on the neutrino mode with more than double the data from their previous update in 2008. Updated results on the antineutrino mode were also presented. What is puzzling is the fact that the best fit value for the mass square difference extracted from the antineutrino mode is 3.36 x 10(-3) eV^2, which is far higher than the value determined using the neutrino mode data, 2.35 x 10^(-3) eV^2. One should note that the result for the neutrino mode was based on 7 x 10^(20) POT while the antineutrino mode result was based on significantly low statistics of 1.71 x 10^(20) POT. If the results turn out to be correct, it could mean CPT violation or the existence of new neutrino interaction. Clearly these data are still very preliminary and we should wait and see how this discrepancy disolve when more data becomes available.

MiniBoone also presented the results for both the neutrino and antineutrino modes. For both modes, an excess in the electron-like events was found in the low energy region. For the neutrino mode, there is an unexplained 3 sigma electron-like excess in the region of energy > 475 MeV, while in the region below 475 MeV the fit does not agree with LSND. For the anti-neutrino mode, there is a small excess above 475 MeV and the fit below 475 MeV agree with LSND result. It would be interesting to see if the low energy excess is still present when more statistics have been accumulated and when we have a better understanding of the background in the low energy region.

For the Neutrino Conference series, it has been the case that there are plenary sessions only. One thing that has been started since Neutrino 2008 in Christ Church, New Zealand, is the poster session. This is not uncommon in physics conferences, but what is special is the 3 min talks that enable the poster presenters to advertise their posters. Given that there are more than one hundred of them, the execution of the "3 min talk sessions" was quite an operation! Fortunately, the sessions went very smoothly, and since the talks were only for 3 min duration each, the speakers really had to get to the point right away making it an efficient way to learn numerous new results.

In addition to the exciting physics, life in Athens has been quite an experience. As you all know, there have been several protests and riots in Athens due to the economic crisis. Today is the third day since the Metro has gone on strike. This has posted a big problem for conference participants as most of us are staying quite far from the Megaron conference center and the Metro is the best way to get there from the hotels especially given that it is extrememly hard to get a taxis in central Athens. Hopefully the strike will end soon, or at the very least that it will not be extended to the airport in the next few days when everyone will be flying out of Athens.

This is it for this post. In the next one, I will report on the theoretical developments and additional experimental updates that I have learned at the conference as well as my overall impressions. So stay tuned.

Lattice 2010, Day Four

2010-06-18T12:07:00.002+02:00

Today's first plenary session was started by Kazuyuki Kanaya with a talk on finite-temperature QCD. Many groups are looking for the transition temperature between the confined and deconfined phases, but since in the neighbourhood of the physical point, the transition is most likely a crossover, the value of the "critical" temperature found may be dependent on the observable studied. There was further some disagreement even between different studies using the same observables, but those discrepancies seem to have gone mostly away.

Next was Luigi Del Debbio speaking about the conformal window on the lattice. The motivation for those kinds of studies is the hope that the physics of electroweak symmetry breaking by originate not from a fundamental scalar Higgs, but from a fermionic condensate similar to the chiral condensate in QCD arising from a gauge theory ("technicolor") living at higher energy scales, perhaps around 1 TeV. One is then motivated to look for gauge theories having an infrared fixed point. Lattice simulations can help studying the question which combinations of N_c, the number of colours, and N_f, the number of fermion flavours, actually exhibit such behaviour. The Schrödinger functional can be used to study such questions, but while there are a number of results, no very clear picture appears to have emerged yet.

The second plenary session of the morning was opened with a talk on finite-density QCD by Sourendu Gupta. QCD at finite density, i.e. finite chemical potential, is plagued by a sign problem because the fermionic determinant can no longer be real in general. A number of ways around this problem have been proposed. The most straightforward is reweighting, the most ambitious a reformulation of the theory that manages to eliminate the sign problem entirely. On the latter front, there has been progress in that the 3D XY model, which also has a sign problem, has been successfully reformulated in different variables in which it does no longer suffer from its sign problem; whether something similar might be possible for QCD remains to be seen. Other approaches try to exploit analyticity to evade the sign problem, either by Taylor-expanding around zero chemical potential and measuring the Taylor coefficients as susceptibilities at zero chemical potential, or by simulating at purely imaginary chemical potential (where there is no sign problem) and extrapolating to real chemical potential. In this way, various determinations of the critical point of QCD have been performed, which agree more or less with each other. All of them lie in a region through which the freeze-out curve of heavy-ion experiments is expected to pass, so the question of the location of the critical point may become accessible experimentally.

The last plenary talk of the morning was Takeshi Yamazaki talking on a determination of the binding energy of helium nuclei in quenched QCD. The effort involved is considerable (there are more than 1000 different contractions for ⁴He, and the lattices considered have to be very large to be able to accommodate a helium nucleus and to distinguish between true bound states and attractive scattering states), even though the simulations were quenched and the valence quarks used corresponded to a pion mass of about 800 MeV. The study found that helium nuclei are indeed bound.

In the afternoon there were parallel sessions.

Lattice 2010, Days Two and Three

2010-06-17T10:03:00.000+02:00

Yesterday was an all-parallels day, so there are no plenary talks to summarise. In the evening there was the poster session.

The internet connection at the resort does not really have the capacity to deal with 360 computational physicist all reading their email, checking on their running computer jobs, browsing the hep-lat arXiv or writing their blog at the same time; this may lead to late updates from me, so please be patient.

Today's first plenary session was the traditional non-lattice plenary. The first talk was by Eytan Domany, who spoke about the challenges posed to computational science by the task of understanding the human genome. A large part of his talk was an introduction to the biological concepts involved, such as DNA, chromosomes, genes, RNA, transcription, transcription factors, ribosomes, gene expression, exons, introns, "junk" DNA, regulation networks and epigenetics. These days, it is possible to analyse the expression of thousands of genes in a sample by means of a single chip, and the data obtained by performing this kind of analysis on large numbers of samples (e.g. from different kinds of cells or from different patients) can be seen as an expression matrix with rows for genes and columns for samples. The difficult task is then to use this kind of large data matrix to infer regulation networks or connections between gene expression and phenotypes. Apparently, there are physicists working in this area together with the biologists, bringing in their computational expertise.

The second plenary talk was an LHC status summary given by Slawek Tkaczyk. The history of the LHC is of course well known to readers of this blog; so far, the first data are being analysed to "rediscover" the Standard Model with the aim of discovering new physics in the not too distant future, but there was no evidence of e.g. the Higgs or SUSY shown (yet?).

The second plenary session was devoted to non-QCD lattice simulations. The first talk was Renate Loll speaking on Lattice Quantum Gravity, specifically on causal dynamical triangulations. This approach to Quantum Gravity starts from the path integral for the Einstein-Hilbert action of General Relativity and regularises it by replacing continuous spacetime with a discrete triangulation. The discrete spacetime is then a simplicial complex satisfying certain additional requirements, and the Wick-rotated path integral can be treated using Monte Carlo techniques. In one phase of the (three-parameter) theory, the macroscopic structure of the resulting spacetime has been found to agree with de Sitter-space. Another surprising and interesting result of this approach has been that the spectral dimension associated with the diffusion of particles on the discrete spacetime is continuously going from around 2 at short (Plackian) to 4 at large distances.

Next was a talk on exact lattice SUSY by Simon Catterall. Normally, a lattice regularisation completely ruins supersymmetry, but theorists have found a way to formulate certain classes of supersymmetric theories (including N=4 Super-Yang-Mills) on a special kind of lattice, giving a local, gauge-invariant action with a doubler-free fermion formulation. This may offer a chance to study quantum gravity by simulations of lattice SUSY via the AdS/CFT correspondence.

In the afternoon there were excursions. I had signed up to the only excursion for which places were still available, which was a tour of a Sardinian winery with a wine tasting. The tour was not too interesting, as everything was very technologically modern, and as somebody said, we can go and look at the LHC if we want to see modern technology. The wines tasted were very nice, though.

Social programming

2010-06-16T15:02:00.009+02:00

Ever attended a physics conference before? Then you should know that the most important moments are neither the plenary nor the parallel sessions, but... coffee breaks! Presentations at plenary sessions are often funny and entertaining; talks at parallel sessions is where one can really learn the details of any remote analysis. But for the gory details, the internal gossips, the questions one does not dare to ask in public, and everything that cannot be said in front of a formal audience but that is important to get to know, coffee-break-chats are what one needs. Alternatively, one can similarly profit of being seated at the right table during the conference dinner, or to participate to the right social events.

Now, this year ICHEP is proposing an impressive social program: apart for the usual welcome reception, we will get a concert in the Salon of the "Hôtel de Ville" of Paris on Friday, we can choose among several different visits of Paris on Sunday, we will be able to visit the Grande Galerie de l'Évolution on Tuesday and have dinner there, not to speak of the Nuit des particules. The conference secretariat just emailed me, asking to choose two of the different visits scheduled on Sunday. I have been in Paris more than once and already visited all the locations the program is proposing, so I guess I should instead choose according to a conference-coffee-break criterion. Where would I be able to come across the most succulent gossips? Where will I most likely learn the most by informally chatting during the visit? What would be the spot to attract the conference VIP's? Tour Eiffel? Opéra de Paris? The Jacquemart-André Museum? The "Cité des Sciences"? The Marais neighborhood? The Panthéon? Where do you plan to go?

Lattice 2010, Day One

2010-06-14T21:41:00.002+02:00

Hello from the Atahotel Tanka Village Resort in Villasimius, Sardinia, Italy, where I am at the Lattice 2010 conference. I know that this blog is called "Blogging ICHEP 2010", but that doesn't mean I can't blog from another conference, does it?

The conference started this morning with a talk by Martin Lüscher about "Topology, the Wilson flow and the HMC algorithm". It is by now well known in the lattice community that Monte Carlo simulations of lattice QCD suffer from a severy problem with long autocorrelations of the topological charge of the gauge field. This problem affects the HMC algorithm and its variants that are used in lattice simulations with dynamical fermions just as well as the simple link updating schemes (Metropolis, heat bath) that can be used for pure gauge or quenched calculations. The autocorrelation time of the topological charge grows roughly like the fifth power of the inverse lattice spacing a as a is taken to zero. This is a real problem because it indicates the presence in the system being simulated of modes that are updates only very slowly, and as a consequence the statistical errors of observables measured from Monte Carlo simulations may be seriously underestimated, because the contribution to the error coming from the long tails of the autocorrelation function that stem from those modes are not properly taken into account. Martin Lüscher then introduced the Wilson flow, which is an evolution in field space generated by the Wilson plaquette action, and which can in some sense be seen as consisting of a sequence of infinitesimal stout link smearings. For the case of an abelian gauge theory, the flow equation can be solved exactly via the heat kernel, and it can be shown that it gives renormalised smooth solutions. For QCD, the same can be seen to be true numerically. Defining a transformed field V(U) by running with the Wilson flow for a specified time t₀, it can then be shown that the path integral over U is the same as the path integral over V(U) with an additional term in the action that comes from the Jacobian of the transformation and is proportional to g₀/a times the integral of the Wilson plaquette action along the flow trajectory. As a goes to zero, the latter term will act to suppress large value of the plaquette. An old theorem of Lüscher shows that the submanifold of field space with a plaquette values less than 0.067 divides into topological sectors, and hence the probability to be "between" topological sectors decays in line with the suppression of large plaquettes by the g₀/a term. This explains the problem seen, but also offers hope for a solution, since one might now try to develop algorithms that make progress by making large changes to the smooth fields V.

This was followed by two review talks. The first was a review of the state of the art in hadron spectroscopy and light pseudoscalar decay constants by Christian Hölbling emphasizing the reduction of systematic errors achieved by decreasing lattice spacings and pion masses and increasing simulation volumes.

The second review talk of the morning was given by Constantia Alexandrou, who reviewed hadron structure and form factor calculations from the lattice, drawing attention to the many remaining uncertainties in this important area, where in particular the axial charge g_A of the nucleon is consistently measured to be significantly lower on the lattice than in nature.

The last plenary speaker of the day was Gregorio Herdoiza, who spoke about the progress being made towards 2+1+1 flavour simulations. The collaborations currently pursuing the ambitious goal of including a fully dynamic charm quark in their simulations are ETMC and MILC. MILC is using the Highly Improved Staggered Quark (HISQ) action to reduce discretisation errors, whereas ETMC is relying on a variant of twisted mass fermions with an explicit breaking of the mass degeneracy for the strange/charm doublet. In the former case, the effects of reduced lattice artifacts are clearly seen, while in the latter case the O(a²) mass splitting between the neutral and charged pion increases with the number of flavours. In either case, a significant effort is necessary to tune the strange and charm quark masses to their physical values, but the effort is definitely well-spent if it leads to N_f=2+1+1 predictions from lattice QCD that include all effects of an active charm quark.

In the afternoon there were parallel talks. Two that I'd like to highlight were the talk of Bastian Knipschild from Mainz, who presented an efficient method to strongly reduce the systematic error on nucleon form factors coming from excited state contributions, and David Adam's talk in which he presented a generalisation of the overlap operator to staggered fermions that gives a chiral two-flavour theory.

A Zeptospace Odyssey: Gian Francesco Giudice's Brilliant New Book

2010-06-12T13:32:00.002+02:00

Today rather than discussing what ICHEP 2010 will bring us I have something better to do, which will have a much more sizable positive effect for the diffusion of particle physics than a wish-list of measurements and findings. In fact, I hold in my hands a brand new copy of Gian Francesco Giudice's book, "A Zeptospace Odyssey - A Journey into the Physics of the LHC". All I have to do is to explain to you why you really should buy, read, and give as a present this book to all your friends.

Gian Francesco Giudice

First of all, a word on the author. Gian Francesco Giudice is a brilliant theoretical physicist who has worked at the CERN laboratories in the Theory Division since 1993. His scientific career brought him in many places before that, but it originated in Padova University, the place where I myself studied and now work. He is only five years older than me, but together with my slow academic career they were enough to get me to enjoy him as a teacher in a course on Group Theory during my Ph.D. studies in Padova. Since then, there is respect and friendship among us, although I see him rarely.

Giudice is a clear thinker and the Physics Department in Padova University aches for his escape, but he is always greeted warmly when he visits us. His last visit was two weeks ago, when he gave a very insightful lecture. It was only then that I learned about his book, silly me.

Giudice has authored dozens of important scientific publications. Before I describe his book, let me cite here a few recent ones. To make the list very short, I only pick papers with more than 50 citations produced in the last six years.

- "Towards a complete theory of thermal leptogenesis in the SM and MSSM", with A. Notari, M. Raidal, A. Riotto, A. Strumia . 56pp. Published in Nucl.Phys.B685:89-149,2004, cited 333 times.

- "Split supersymmetry", with A. Romanino. 28 pp.
Published in Nucl.Phys.B699:65-89,2004, cited 319 times.

- "Aspects of split supersymmetry", with N. Arkani-Hamed, S. Dimopoulos, A. Romanino. 51pp. Published in Nucl.Phys.B709:3-46,2005, cited 235 times.

- "The Well-tempered neutralino", with N. Arkani-Hamed, A. Delgado. 29pp.
Published in Nucl.Phys.B741:108-130,2006, cited 68 times.

- "The Strongly-Interacting Light Higgs", with C. Grojean, A. Pomarol, R. Rattazzi. 45pp. Published in JHEP 0706:045,2007, cited 95 times.

The Book: first impressions

The book is a nice-looking hardcover volume, published by Oxford University Press earlier this year. It is not thick enough to scare you away, and once you open it and start turning its pages, you get a feeling of the clean, tidily typeset and clearly readable text. You soon find dozen of beautiful pictures in black and white, few graphs all looking simple to understand, and absolutely no mathematical formulas. This is a book for everybody! But can it, given the subject ?

Yes, the subject: this is a book about the journey that the Large Hadron Collider at CERN has undertaken, a journey inside the smallest distance scales which will hopefully bring us to find new riches, and make humanity wealthier of knowledge on the physical world. Divided in three parts ("A matter of particles", "The starship of zeptospace", and "Missions in zeptospace"), it contains 13 sections whose titles appear indeed arcane: forces of nature, dealing with naturalness, supersymmetry, from extra dimensions to new forces. Can this be a book for everybody after all ?

Of course it can! As I recently argued here, no scientific concept is too hard to explain to a reader willing to make a sincere effort. Only, it takes the most capable writer to do the trick. And Gian Francesco is a sublimely capable writer!

I am probably not the best of judges for what concerns the English prose of the book -I am Italian, just as Giudice is-, but I must say that I find the text exceedingly clear and well written. The author makes a real effort to not only explain in the simplest way, and with plenty of spot-on analogies, all the concepts that he believes are necessary in order to perform this journey, but he manages to make the reading quite enjoyable in the meantime! The book is a real mine of anecdotes intertwined with physics explanations, such that it reads very easily and you absorb a wealth of knowledge in the process almost without realizing it.

Let me make a few examples to explain what I mean here. If you are a reader of this blog you must know that I appreciate analogy as a powerful means to make tough physics concepts understandable; if you are not one, you might get a proof of that by reading my recent explanation of electroweak unification using a cup of chocolate, for instance. Being fond of analogies, I could not help appreciating the witty and fun way Gian Francesco explains complicated mathematics such as renormalization, one of the toughest hurdles in making sense of the theory of quantum electrodynamics:

"Imagine that tomorrow is St Valentine's Day. You and your friend David Beckham go out shopping to buy presents for your respective wives. You enter a store an David chooses for Victoria 30 diamong chokers, 50 emerald bracelets, 60 fur coats plus some other expensive items. He keeps careful track of his expenditures, which total some megabillion zillion euros. You pick up a small bouquet of flowers, whose price isn't marked. In the confusion at the checkout counter, all your purchases are rung up together and the total bill aounts to some megabillion zillion euros. Must you really pay some megabillion zillion euros for a bouquet ? Of course not: all you have to do is take the difference between the total bill and David's share, and you find that you must pay only 19 euros and 99 cents.

Something similar happens in calculations of QED. Most of the results of these calculations are equal to colossal numbers (actually infinity). However, these results do not correspond to measurable physical quantities, as much as the total bill above does not refer to what you must actually pay. Once the result of a physical quantity is appropriately expressed in terms of other physical quantities, colossal numbers are subtracted from each other and the result is a perfectly reasonable small number [...]"

The book contains countless quotes from the actors of the play of XXth century physics. I am also a collector of quotes, yet I was happy to find several that I had never heard before. And Gian is quick also to explain things that other authors overlook, oftentimes by using quotations, some of which are of true historical importance. Take this introduction to the neutrino:

"The name "neutrino" was coined jokingly by Enrico Fermi [...] when, during a seminar in Rome, he was asked if the two particles were the same. "No," replied Fermi, "Chadwick's neutrons are large and heavy. Pauli's neutrons are small and light; they must be called neutrinos." Of course the pun is lost in the English translation: in Italian "neutrino" is the diminutive of "neutron" - "little neutron".

How many of you non-Italian speakers knew this ?

To end this section, I need to mention an accident, which I hope will clarify just how accurate this book is. I wanted to write a paragraph here where I would say that incomplete explanations are a problem of any book which attempts the arduous task of explaining tough science to outsiders. I wanted to make the point that it is virtually impossible to stop and make sense of ALL the crucial concepts that arise in an explanation of physical concepts needed to read back-to-back a 250-page-long book. I had spotted one early on: on page 24, one reads that "... general relativity is not just a reformulation of Newton's theory. It predicted new effects - like the anomalous precession of Mercury perihelion[...]".

Aha! Gian fails to explain what this is here. What the heck is the anomalous precession of the perihelion of Mercury ? A non-physicist reading this sentence might be rightfully upset! ...But does he ? He doesn't. A glance at the Index under "Mercury" will reveal that later in the book, on page 224-225, the mysterious planet is mentioned again. And there, eventually, the diligent reader will finally find an answer to his doubt!

So, I owe apologies to Gian Francesco for having doubted of the completeness of this lean but self-sufficient book.

Errors

No review of a book would be complete without at least an attempt at criticizing its contents. I have read the book, and as I already said I found it clean, well written, and remarkably precise. I have nothing to say about the topics: the book covers the history of physics which lead to today's accelerators, the construction of the monster apparata, all the important goals of the LHC, not sparing even the most complex, cutting-edge theories of new physics. However, as they say "there's always one more bug". So let me have a shot at it. My list will be exceedingly short.

1. On page 27, talking of protons accelerated by the Bevatron in 1955, Gian says that "The proton energy was enormous for those days, but is actually less than a thousandth of the energy of a single LHC beam." Of course, he means "the energy of a proton in a LHC beam". The energy of a single LHC beam is trillions of times larger, being contributed by billions of protons.

2. It feels bad to criticize a very well-compiled Index, which is 20-pages long and is a quite useful addition to such a quotation-full book. However, the duty of the reviewer forces me. On page 276 the Index cites a "Wiloczek, Frank" which three lines above is correctly reported to reference two different pages as "Wilczek, Frank". The reference to page 71 should be thus added three lines before, and this line deleted. To be frank (with a lowercase f), the page-71 reference to Wilczek is correctly appearing on page 268, in the reference to "Nobel Prize".

3. As I already mentioned, the English in the text is quite correct and flowing. This is not surprising, given that if you heard Gian Francesco talk you might well exchange him for a Brit (the lack of typical Italian straggling of words in his pronunciation is remarkable). Even commas are used appropriately, following the so-called "Harvard comma" rule for serial lists (a comma is due before the last "and"). However, I found an inconsistent use of British versus American English spelling. On page 10 we read the word "color" and just one line below the word "odour". Who cares, you might ask! True, who cares. But since even 2000-strong scientific collaboration end up arguing at such level of detail on their drafts of scientific publications, I thought I would mention it...

A short interview with the author

Gian was kind enough to answer a few questions on his book for this blog. Here are five questions I posed, and his answers (in Italics):

1 - I am curious to know what brought you to the idea of this book, because I had no previous recollection of popularization activities on your part. What played a major role in deciding to write it: opportunity (being the right person in the right place and with the right means to write a very good account of the LHC adventure), desire to involve more people in the science we do, a challenge with yourself ? Or something else ?

It all started with some public lectures I gave on LHC physics. It was a surprise for me to see first hand how so many people, even those with no physics background, are sincerely fascinated by this wonderful scientific adventure. But at the same time I was really taken aback to see how the media coverage and newspaper articles were grossly misrepresenting the real aims of the LHC, feeding wrong information to the public. Explaining our research activity in simple and accessible terms does not necessarily require being scientifically inaccurate. So I decided to tell the story from a physicist's point of view. Any tale is more enticing when narrated by someone who is participating in the story.

2 - The text is quite clean and devoid of complications - there are no formulas, no graphs, and even the use of scientific notation for numbers is introduced by an apology. When you wrote your book did you aim at the widest possible audience, or was your choice of material rather driven by a specific target (such as, by means of example, high-school students) ?

The book is directed to anyone who is curious about the world of particle physics and the LHC. I made an effort to avoid technical terms and make sure that readers with no physics background could follow the story. But hopefully even physicists (especially young physicists) working at the LHC might find in the book elements that can help them broaden their views on their experiment and their field.

3 - Did anybody help you find and choose the dozens of appropriate quotations that are used in the book to introduce chapters and even subsections ? Their breadth is remarkable.

I like to read and this helped me. The quotations I collected from various authors and disciplines are mostly meant to bring a touch of irony in the presentation of a scientific field that most people believe to be high-browed and arcane. But they are also meant to show how physics is intertwined with other creative, artistic and speculative human activities.

4 - I know very well that you are a busy scientist, and your time is precious. Decreasing your involvement in your studies is probably out of the question, even for a noble goal like that of writing a popularization book on particle physics. How long did it take you to write this book ? Did you take a leave from work to finish it, or did you overburden your summer vacations, or did you instead proceed slowly by using your spare time in tiny bits ?

Finding the time to write was the most serious difficulty I had to face. The project took me about a year, and nights and weekends were my favourite writing periods. My family has been very kind and patient during that time.

5 - Although any book like yours is one of a kind, there always exist similarities in the way they are constructed, or in some choices like the material covered or the depth with which topics are discussed. Did you find inspiration in any previous book by particle physicists? Is there a model you followed ?

I read many books related to particle physics before and during the writing, and I learned much from them. Many of these great books have certainly influenced the way I view the development of our field and the meaning of the LHC. I absorbed this material, but I don't think I followed one particular book or author as a model. Instead I tried to adopt a writing style which is just a crossover from the style of my seminars and public lectures. I like to link ideas and developments in science with their historical context and to present advanced concepts in theoretical physics using simple and familiar analogies.

What others think

If you got this down in my review, you will no doubt have gotten the impression that I was paid very well for my lip service! No kidding: the fact is, my salary was a copy of the book, and my reward in writing the review was ... writing the review! I did not need to lie in the least. But to show you that I am in good company in appreciating Giudice's work, below I attach a short list of reviews on Giudice's book by unsuspectable arbiters.

"Gian Giudice has, as one would expect from such a clear and original thinker, produced a book which both challenges and excites, providing fresh insights into the domain of particles and their interactions. " - Ken Peach, University of Oxford and Royal Holloway University of London.

"This fascinating book is entertaining and comprehensible, leading the reader to the world of extremes: the high technology of the Large Hadron Collider at CERN and its huge particle detectors, the quest for the Higgs particle and the mysterious Dark Matter, and the theories of superstrings and extra dimensions at the verge of human imagination." - Thomas Lohse, Humboldt University, Berlin, Germany.

"I enjoyed this book tremendously. The weaving of important information with fun facts and anecdotes was awesome." - Savas Dimopoulos, Stanford University.

"This book shows that it is possible to describe to non-experts the frontiers of modern physics, in a way which is both faithful and comprehensible. I almost envy the author his right-on-the-bull's-eye explanatory metaphors. I believe that this book will become required reading for anyone interested in the reality of our world and in scientific human endeavour." - Riccardo Barbieri, Scuola Normale Superiore, Pisa, Italy.

"Gian Giudice provides a comprehensive introduction to the LHC as only a physicist working in the field could do." - Lisa Randall, Harvard University.

"Gian Giudice has given us a charming, comprehensive, and deep yet easily readable description of the history, technology, and scientific aspirations of the Large Hadron Collider, perhaps the greatest scientific experiment ever." - Gordon Kane, University of Michigan.

"This book, written by one of the leaders of the field, has a number of outstanding qualities: it is brilliant, original, comprehensive, entertaining and clear. It is a must for cultivated, non specialist readers who want to get an introduction to contemporary particle physics and to the exciting programme of the Large Hadron Collider of CERN." - Guido Altarelli, University of Rome and CERN.

"Gian Giudice has drawn on his deep understanding of physics to write a wonderful book, presenting the central ideas underlying the grand intellectual adventure of particle physics in an engaging and thought-provoking way. A must read for anyone who wants to understand the big questions we face in fundamental physics, and the ways we are tackling them." - Nima Arkani-Hamed, Institute for Advanced Study, Princeton.

A final advice

Buy the book. Read it, appreciate it, and then buy more copies as a gift for friends and relative of yours who think they would not understand particle physics for the life of them. They will be grateful!

Physics at the LHC – What did Guido mean!?

2010-06-09T09:51:00.001+02:00

My summer has started. And the first stop is the conference Physics at the LHC 2010 conference (or pLHC as it has been known internally by the experiments).

As Barbara already mentioned, this conference, occurring this week in Hamburg, Germany, is a preview of results we might expect at ICHEP.

It is Wednesday morning and all the LHC experiments have big plenary talks discussing overviews of their physics current results. One of the CMS results has already been discussed by Tommaso. The results all show the first time the experiments are gingerly dipping their foot into the pool of physics. Ok, so it is the shallow end right now. Observation of many quark-onia bumps, but some real new measurements not done at the LHC’s new higher-than-ever energy, 7 TeV. But they also include the first observation of the W boson and the Z boson at the LHC. Now we’re talking!

One fascinating conversation occured right at the start of the meeting. It was a discussion that occurred during the introductory talk by Guido Tonelli, the spokesperson for CMS (video available). Sitting in the audience was Steven Myers from CERN, who runs the LHC (who also gave a talk on the present and near future of the LHC machine). Both Guido and Fabiola Gianotti (ATLAS spokesperson) pleaded with Steve to give them more luminosity. Guido went one step further and said that if they can give CMS enough luminosity in the next month they will measure the Standard Model for ICHEP.

What does that mean? Both experiments have observed the W boson now. Does that mean measuring its cross section? Or an observation of top quark production? Is there any reasonable plan for the LHC that would give it enough luminosity to observe top quark production!? Or was it just a flashy statement made to urge Steve to give the experiments more data? Does CMS have something up their sleeve? I guess time will tell… And both CMS and ATLAS will do push their data as far as they can.

Update: at the end of the CMS talk, slide 54, there is the statement “100 nb-1, 380 W and 35 Z”, and the speaker said “this we can be sure of.” So, that gives the scale of the CMS plans. And lets hope the LHC can actually give us that 100 np-1 of data!

Late to the party…

2010-06-09T09:09:00.002+02:00

Hi all! We are now about 6 weeks out from the start of ICHEP, and this is my first blog post. So introductions are in order. I’m a professor at the University of Washington, located in fantastic Seattle (we only tell people it is raining…). I’m an experimentalist working on the DZERO experiment at the Tevatron and the ATLAS experiment at the LHC.

I maintain another blog (sporadically). If you read through it you’ll quickly discover my main interests. I love high-pT physics. DZERO’s recent B’s mixing result (see Jester’s earlier posts on it here and here) is fascinating, but sadly aren’t things I work on. I tend to be more interested in top, Higgs physics, and, recently exotic physics. On the flip side I’ve long been fascinated with computing in HEP, having started in high school programming a LeCroy FASTBUS microcontroller (yeah, yeah, tell me about it, I didn’t get out much).

This year’s ICHEP should be special for the LHC and the Tevatron experiments. I know this because of how busy everyone is in the experiments… More on that in a future post.

Physics at LHC

2010-06-07T23:36:00.004+02:00

This week some 270 people are getting a taster of what ICHEP may (or may not) be like: at Physics at LHC, a conference that started today at DESY in Hamburg, Germany, about, well... physics at the LHC. I've heard some hints that different results will be available for the summer conferences, seen many interesting talks and heard many excited scientists getting pure bliss out of the fact that they can finally present real data from real collisions at a really remarkable LHC.

If you're not in Hamburg you can still follow the action -- the talks are webcast and also recorded and put up on the conference's indico page along with the slides.

Enjoy!

News From The Third World Of Research - Italy

2010-06-04T16:47:00.004+02:00

Economy, and all the more so politics, should be of no interest to a focused researcher in fundamental Physics, in an ideal world. But we do not live in an ideal world.

After two years spent saying that Italy has a strong economy and is doing better than the rest of Europe, and strongly criticizing whomever tried to warn that the economical crisis was not over yet, the Italian government led by Silvio Berlusconi has made a sharp turn. The buzzword is now "avert the Greek risk", and while painting dreadful scenarios Berlusconi and his ministers have crafted a finance law that drags over 30 billion euros mostly from salaries. The anti-Robin-Hood strikes again.

The thing would be sad by itself, but some of the ancillary rules contained in the new law will have a devastating effect on basic research. I can only speak for particle physics, where I know how funds are spent. For the most part, funds in particle physics research are administered quite well in Italy, a result of the narrow margin within which researchers have to manouver; there are cases of abuse, but these are rare. Let us take the case of the participation to CERN and its experiments.

In order to participate in the large experiments at the CERN laboratory, over 1000 researchers based in Italian universities need to periodically travel to Geneva. Actually, the participation to the experiments forces researchers to make themselves available for week-long shifts at the data-taking, plus of course performing maintenance to the detector components they themselves built. Then there are meetings, working groups, etcetera -but these are not crucial activities, since they can be performed through remote videoconferencing.

Now, what does the new finance law says ? It says that effective June 1st, the per-diem compensation of researchers traveling abroad (some 120 euros per day, with which one should pay for lodging, meals, and all the rest) is zeroed. Only lodging expenses are refunded, and we do not yet know whether meals will be in some way paid back. Furthermore, the law states that the total expense of institutes such as INFN -the Italian Institute of Nuclear Physics that hired me- for missions abroad cannot exceed 50% of what was spent last year. Since we are in June, you can well understand how narrow a margin this leaves for travel abroad in 2010!

Now, since without Italian researchers the CERN experiments cannot run safely, unless they overburden with shifts the scientists from other countries, the matter poses a urgent problem. In principle, one might think that physicists do not work for a salary, but for the beauty of science: this would not be far from the truth in the case of Italian physicists, since their salary is about a third of that of our colleagues from the US, Germany, or many other countries. So we can expect that Italian researchers will bow their head and continue working abroad even if they spend more than what they earn. But I have my doubts.

The matter is made complicated by the fact that INFN promised quite a bit of support, in terms of available manpower, to the CERN experiments. These agreements will go unattended if the Italian government does not repair the awkward situation.

UPDATE: two links.
The first link is to Peppe Liberti's blog, who translates in Italian part of the text above in a post on the same topic.
The second is the original text of the law (in Italian only, sorry). You can find the part of relevance to researchers traveling abroad in section 6, subparagraph 12.

Up and down

2010-05-30T18:36:00.006+02:00

The Standard Model works so well that you wish sometimes that we had not designed such an efficient theory. So where to look for excitement, since everything seems so much and so boringly in control ? Where does New Physics hide, waiting for us to be discovered ?

"LHC" seems the most obvious answer -- test the least-understood sector of the theory (the breaking of electroweak symmetry) by trying to produce the missing piece of the Standard Model (the Higgs boson) and/or hopefully something else, and unexpected. We all hope to be in Rabi's position, who exclaimed "who ordered that ?", when the muon was discovered and did not fit into anybody's plans. So should we just wait for the LHC to give all the answers ?

Fortunately not. Jester's posts show at least one place where things are moving fast, if not always in the direction of your dream -- flavour physics, i.e. the electroweak transitions under which a quark or a lepton changes from one type to another. Obviously, this is not an easy game. Quarks are subjected to strong interaction -- and thus confined into hadrons, so that you never actually observe the decay of a naked quark into another one, but rather that of a meson to one or several others. The computation of such processes is a theorist's nightmare, involving hadronic quantities that are very hard to estimate and possibly hiding faint contributions from New Physics. Leptonic processes are much cleaner, and provide signals of New Physics almost completely free of Standard Model backgrounds, like $\mu\to e\gamma$. But it is a "hit or miss" game : you need New Physics to make such processes frequent enough so that you can detect them... but you have no clue if this will be indeed the case.

The power of flavour physics lies in its sensitivity to virtual effects : by measuring accurately this process, you are sensitive to quantum corrections induced by particles/interactions at much higher energy scales. The past history has proved that it was not only theorists' daydreaming : the non-observation of $K_L\to\mu\mu$ led to the introduction of charm, the violation of the CP symmetry in the kaon system suggested the existence of a third generation of quarks and leptons, and neutrino oscillations were the first hint of physics beyond the Standard Model with the need for right-handed neutrinos. Moreover, despite our difficulties with the strong interactions, we can make (sometimes) quantitative statements : for instance, the upper limits on the difference of masses in the $B\bar{B}$ system showed that the top quark had to be much heavier than previously thought...

The present is a reflection of the past, as the philosophers say. Flavour physics keeps being a stringent constraint on many models of new physics, since they often predict transitions at much higher rates than observed. $\epsilon_K$, $b\to s\gamma$, electric dipole moments and so on have proved stubborn killers for unsuspecting theories for what lies beyond the Standard Model. They are indeed so powerful that for most theories, they push the scale for New Physics much above 1 TeV to make their impact negligible on the processes currently observed... Eh but wait a minute ! This is not what we want -- we want New Physics at the TeV scale !

Fortunately, you can find specific scenarios (such as Minimal Flavour Violation) to accomodate both the data from flavour physics and our LHC expectations. But it would be far more natural (historically speaking) and exciting (scientifically speaking) to see hints of new physics already in some flavour processes, before actually producing new particles at LHC responsible for these deviations. Hence all the recent discussions on the CDF/D0 results, taking back from one hand (CDF $\phi_s$) what they gave us from the other (D0 $A_{SL}$). That is the fundamental problem of flavour physics: since you do not observe new particles, you must spend your time on assessing how much your process has drifted from the Standard Model expectation. And it is a cruel game. Among the many deviations from the Standard Model observed in flavour physics, the majority stays in the grey area between 2 and $3\sigma$ before fading slowly into 1 $\sigma$ oblivion after a more careful scrutiny, both from experimentalists and theorists... Who is still excited by the $b\to s\bar{s}s$ processes, which used to deviate at 3 $\sigma$ from the Standard Model expectations and are now in perfect agreement with it ?

Are the recent results from CDF and D0 two different points in these Russian hills, or will one of these anomalies stay with us to finally be confirmed as a real effect ? Only time -- and experiment ! -- will tell.

Telescope transport now leaving from gate A...

2010-05-29T22:09:00.006+02:00

Out on the job market in the real world, physicists are popular people. They can get almost any job because, well, they can do almost any job. They are good at maths, computing, logic, solution-oriented and creative thinking. They are used to working in teams, in different languages, with different cultures, so when physicists decide not to stay in physics, they might end up as investment bankers, software developers, patent lawyers, safety specialists, teachers, consultants. And probably many other professions that I’m not even aware of.

But it’s not only in life after physics that their versatility is useful. As particle physicist you are as much a brainy boffin as you are a technician with screwdriver in hand, or a manager, or an accountant – and sometimes you can be a truck driver. Tomorrow morning at 8:30 a special kind of van will leave the German lab DESY and go to CERN to deliver a beam telescope. At the wheel: coordinator and physicist (and blogger) Ingrid Gregor…

The beam telescope is about as versatile as the people who use it. You need it to check whether what you think you see with your detector really is there, a sort of cross-check mechanism, only with lots of added value. In labs around the world, scores of physicists are already working on new generations of particle detectors – like the ones busy taking data at the LHC at CERN right now. These need to be tested, and the best way to test a particle detector is to put in a beam of particles, a test beam. There are test beams at all the major labs around the world, including DESY and CERN – particles circulating in the accelerators are directed to separate areas where many smallish experimental stations receive them. Different accelerator deliver different particles, and depending on the type of tests you want to make some are more useful than others.

The beam telescope – the product of the European ‘EUDET’ project that coordinates detector development infrastructures – gets into the ring with the test detectors (all sorts of different kinds, by the way) and tests their precision. A highly precise tool itself, its six boards, fitted with silicon pixel sensors, can measure the beam particle tracks to a precision of three micrometres, while another device, typically pixel or strip detectors, can be positioned in the middle to be studied. (I stole this sentence from a NewsLine story in case you’d like to read up on the telescope). It goes to CERN for a round of shifts with ATLAS detector upgrades and has an extremely full agenda at CERN until the end of November, when Ingrid gets back into the telescope truck and hauls it back to DESY.

And now the best bit: I’ll be part of the delivery! I’ll be driving down with Ingrid, Volker the technician, EUDET the telescope and a stack of freshly made CDs…it’s a twelve-hour trip after all. You’ll be hearing more of it later!

Important update from CDF

2010-05-29T10:40:00.004+02:00

Last week the D0 experiment at the Tevatron presented the new measurement of the same-sign dimuon charge asymmetry in B-meson decays. This asymmetry probes CP violation in B-mesons, including the $B_s$ mesons that have been less precisely studied than their $B_d$ friends and may still hold surprises in store. D0 claimed that their measurement is inconsistent with the standard model at the 3.2 sigma level and hints to a new physics contribution to the $B_s \bar B_s$ mixing. 3 sigma anomalies in flavor physics are not unheard of, but in this case there were reasons to get excited. One was that the $B_s$ system is a natural place for new physics to show up, because the standard model contribution to the CP-violating mixing phase is tiny, and theoretical predictions are fairly clean. The other reason was that the D0 anomaly seemed to go along well with earlier measurements of CP violation in the $B_s$ system. Namely, the measurement of CP violation in the time-dependent tagged $B_s \to J/\psi \phi$ decays displayed a 2.1 sigma discrepancy with the standard model, and some claimed the discrepancy is even higher when combined with all other flavor data. In other words, all measurements (except for $B_s \to D_s \mu X$ that however has a larger error) of the phase in the $B_s \bar B_s$ mixing consistently pointed toward new physics.

Disappointingly for most theorists, another Tevatron's experiment CDF recently presented an update that reverses that trend. CDF repeated the measurement of $B_s \to J/\psi \phi$ on a larger data sample of 5.2 inverse femtobarn, that is with 2 times larger statistics than in the previous measurement. The new result is consistent with the standard model at the 0.8 sigma level:
So at this moment only one experiment claims to see an anomaly in the $B_s$ system, while another measurement of the $B_s \bar B_s$ mixing phase is perfectly consistent with the standard model. Of course, further measurements of the mixing phase may bring another twist to the story...hopefully, new illuminating experimental results will be presented at ICHEP 2010.

What should we expect from LHC?

2010-05-26T21:51:00.006+02:00

June is coming, summer conferences are approaching, LHC physicists are feverishly working to produce results to show.

In the next few months there will be three main conferences where physics results from the LHC experiments will be presented: the nearest one is Physics At LHC, that will take place at Desy in Germany the second week of June; the second one is, erm... you know... ICHEP; the third one is the Hadron Collider Physics Symposium in Toronto, at the end of August. The kind of results one might expect to be presented at each of these conferences is rather different. The LHC is in fact steadily delivering proton-proton collisions at 7 TeV: the farther in time the conference, the more integrated luminosity the experiments will be able to use for their analyzes.

Could we try to guess what is likely to be shown at ICHEP by ATLAS and CMS? Well, it's definitively not an easy prediction: even assuming a perfect efficiency of the two experiments in collecting the data and analyzing it, the LHC beam conditions are improving every day, and the exploitable integrated luminosity at - let's say - mid July can largely vary.

Let's then try first a different exercise: which results are more likely to be seen at a conference as a function of the integrated luminosity collected at 7 TeV, from the small amount we already know as been secured by the experiments to the 1 fb^-1 promised by the machine for the end of the 2010-2011 running? Warning: what follows is a very approximate list, I might have missed important signals here and there, and my judgment is certainly biased by my ATLAS experience. Here's what we'll get (or what we already got):

10-100 μb^-1: millions of charged pions to happily redo the charged multiplicity analysis published with the 900 GeV data collected in 2009; a few tens of $J/\psi \to \mu \mu$, a few jets here and there. Any resonance that can be spot using the tracker system (like K's and $\Lambda$'s) has been been seen at this point; signal from $\pi^0$ and $\eta$ decaying in photons pairs is found and well isolated.
100-1000 μb^-1: any hint of a $J/\psi \to \mu \mu$ peak should now be clearly visible;
1-10 nb^-1: more jets. And of course more jets-related measurements.
10-100 nb^-1: a few tens of W begins to appears in the data. The lucky ones might have seen a few Z bosons. A first observation of prompt inclusive electrons should be at reach at this point.
100-1000 nb^-1: more and more jets. The first inclusive muon measurements should be feasible. Signal from prompt photons should have been isolated.
1-10 pb^-1: at this point ATLAS and CMS should have secured enough W and Z to dare to attempt a first cross-section measurement. They might be able to pretend to have seen the top quark.
10-100 pb^-1: first B-physics related measurements. Something could already be said about some exotic scenarios, and some SUSY points.
100-1000 pb^-1: at this point, one could even optimistically hope in some timid news about the Higgs boson (exclusion), at least where the sensitivity is higher.

Where do we stand today? ATLAS and CMS are today around point 4. (more around the 10 nb^-1 lower end, anyway), and that kind of results will most likely be shown at Physics At LHC together with a lot of performance studies. The question is then: how much more luminosity will the machine be able to deliver before ICHEP? Since this post is already long enough, I will postpone my educated guesses to the next ones. Stay tuned.

Of things to come

2010-05-26T16:17:00.004+02:00

"Predictions are always difficult, especially when they concern the future" is a quote that is sometimes attributed to Niels Bohr, but which in any case is quite true. Still, I'll hazard a few predictions about what we'll see at this summer's conferences: At ICHEP 2010, we'll see the first results from the LHC, but there will almost certainly be no indications for new physics yet from that direction. The indications of BSM physics will come instead once again from precise measurements in flavour physics, such as the decays of B_s and D_s mesons. At the LATTICE 2010 conference, we'll se further progress in tackling quantities that have been difficult to treat on the lattice so far, but again the most immediately interesting results will likely be those that reduce the statistical and systematic errors on predictions of quantities of relevance to flavour physics. In fact, I'll even dare to suggest that this prevalence of flavour physics results in hinting at, and possibly revealing, BSM physics is a pattern that's likely to extend further into the future. I won't go so far as to suggest that the LHC won't see direct evidence for new physics at some point, but given the uncertain signatures of new physics when taken in conjunction with the difficult nature of the background at a hadron collider I'd expect that we are more likely to see that precise measurements of flavour observables give conclusive evidence of BSM physics than we are to see, say, the direct production and subsequent detection of superpartners at the LHC. But that's just my opinion as a theorist interested in heavy-quark physics, so there may be some wishful thinking involved.

A perfectly workable unit

2010-05-24T18:10:00.002+02:00

I'd like to introduce a new unit to the already rather colourful list of physics units (what do I mean, colourful? Well, what sort of a unit do call an 'inverse femtobarns'?) - the unit of 'big things to come'. It permeates my to-do list and determines absences, presences and social engagements. Last year's 'big things to come' were several orders of magnitude more abundant than those of previous years (though it never feels like nearing zero) - there were colliders to start back up, first collisions at various energies, anniversary celebrations, open days, First Physics days, exhibitions and finally an important political visit paired with a huge celebration. Now, amazingly, this is all over and it is time to cross done to-dos off the list and face the next big thing to come. For me, that is ICHEP.

I am not nearly as busy preparing for as some of my colleagues are, but it is there on the horizon as a week in Paris that might bring interesting developments for the ILC, maybe exciting news from LHC experiments, and definitely new shores in terms of particle physics communication. We'll tell you more about those later...

And CMS, in the meantime...

2010-05-19T15:32:00.002+02:00

The blogosphere is abuzz with the recent news of a startling new result by DZERO -see the previous post by Jester on this issue by scrolling down- but in the meantime at CERN experimenters are quietly working at their first meaningful physics results, with proton-proton collisions at the highest energies so far achieved.

This morning, after months of work, finally a paper by the CMS collaboration sees the light. Or should I say the pre-light, since the paper has been sent to the Cornell Arxiv, and to Physical Review Letters, but it is so far only a pre-print: the PRL reviewers will need to approve it for publication. Given the excruciatingly long and painful internal review process that the paper has withstood within CMS, by about 5000 eyes (or 2500 pairs if you prefer), I would say there is not a chance that the paper does not pass the standards of PRL now. But maybe it is better to be cautious, so - pre-light!

The paper reports on a measurement of Bose-Einstein correlations between pairs of charged pions recorded by CMS in its early runs at 0.9 and 2.36 TeV of center-of-mass energy. Nothing too exciting, but indeed a nice clean new measurement from data which allows little else at this stage, due to the small luminosity collected by the LHC this far.

I was personally involved in the analysis of the data for this result, so I quite well know how painful it was to produce the paper. But it has left today, so it is time to cheer up. In the meantime, CMS is working at dozens of other publications, which are expected in time for ICHEP. Paris will be brimming with new LHC results, I am sure. Another reason to look forward to it, besides the ephemeral albeit exciting news from the DZERO detector!

New Physics Claim from D0!

2010-05-17T16:29:00.014+02:00

Tevatron not dead, or so it seems. Although these days all eyes are turned to the LHC, the old Tevatron is still capable to send the HEP community into an excited state. Last Friday the D0 collaboration presented results of a measurement suggesting the standard model is not a complete description of physics in colliders. The paper is already available at this link, and it should be out on arXiv tonight.

The measurement in question concerns CP violation in B-meson systems, that is quark-antiquark bound states containing one b quark. Neutral B-mesons can oscillate into its own antiparticles and the oscillation probability can violate CP (much as it happens with kaons, although the numbers and the observables are different). There are two classes of neutral B-mesons: B_d and its antiparticle B_d where one bottom quark (antiquark) marries one down antiquark (quark), and B_s, B_s with the down quark replaced by the strange quark. Both these classes are routinely produced Tevatron's proton-antiproton collisions roughly in fifty-fifty proprtions, unlike in B-factories where mostly B_d B_d have been produced. Thus, the Tevatron provides us with complementary information about CP violation in nature.

There are many final states where one can study B-mesons (far too many, that's why B-physics gives stomach contractions). The D0 collaboration focused on the final states with 2 muons of the same sign. This final state can arise in the following situation. A collision produces a b \bar b quark pair which hadronizes to B and B mesons. Bottom quarks can decay via charged currents (with virtual W boson), and one possible decay channel is b -> c μ^- ν_μ. Thanks to this channel, the B meson sometimes (with roughly 10 percent probability) decays to a negatively charged muon, B -> μ^- X, and analogously, the B meson can decay to a positively charged antimuon. However, due to BB oscillations B-mesons can also decay to a "wrong sign" muon: B -> μ⁺ X, B -> μ^- X. Thus oscillation allow the BB pair to decay into two same sign muons a fraction of the times.

Now, in the presence of CP violation the B->B and B->B oscillation processes occur with different probabilities. Thus, even though at the Tevatron we start with the CP symmetric initial state, at the end of the day there can be slightly more -- than ++ dimuon final states. To study this effect, the D0 collaboration measured the asymmetry

A_sl^b = (N_b⁺⁺ - N_b^--)/(N_b⁺⁺ + N_b^--) .

The standard model predicts a very tiny value for this asymmetry, of order 10^-4, which is below the sensitivity of the experiment. This is cool, because simply an observation of the asymmetry provides an evidence for contributions of new physics beyond the standard model.

The measurement is not as easy as it seems because there are pesky backgrounds that have to be carefully taken into account. The dominant background comes from ubiquitous kaons or pions that can sometimes be mistaken for muons. These particles may contribute to the asymmetry because the D0 detector itself violates CP: due to budget cuts the Tevatron abandoned construction of the D0 detector made of antimatter. In particular, the kaon K⁺ happens to travel further than K^- in the detector material and may fake the positive value of asymmetry.

At the end of the day, after (hopefully) carefully subtracting the backgrounds, D0 quotes the measured asymmetry to be

A_sl^b = -0.00957 ± 0.00251(stat) ± 0.00146 (syst),

that is the number of produced muons is larger than the number of produced antimuons with the statistical significance estimated to be 3.2 sigma. The asymmetry is some 100 times larger than predicted by the standard model!

Of course, it's too early to celebrate the downfall of the standard model, as in the past the bastard have recovered from similar blows. Yet there are reasons to get excited. The most important one is that the latest D0 result goes well in hand with the anomaly in the B_s system reported by the Tevatron 2 years ago. The asymmetry measured by D0 receives contributions from both B_s and B_d mesons. The B_d mesons are much better studied because they were produced by tons in BaBar and Belle, and to everyone's disappointment they were shown to behave according to the standard model predictions. However BaBar and Belle didn't produce too many B_s mesons (their beams were tuned to the Upsilon(4s) resonance which is a tad too light to decay into B_s mesons), and so the B_s sector can still hold surprises. Two years ago CDF and D0 measured CP violation in B_s decays into J/ψ φ, and they both saw a small, 2-sigma level discrepancy from the standard model. When these 2 results are combined with all other flavor physics data it was argued that the discrepancy becomes more than 3 sigma. The latest D0 results is another strong hint that something fishy is going on in the B_s sector.

Both the old and the new anomaly prompts introducing to the fundamental lagrangian a new effective four-fermion operator, ∼ (b s)² + h.c., that contributes to the amplitude of B_s B_s oscillations. At this point we have no hints whatsoever what could be the source of this new operator, and the answer may even lie beyond the reach of the LHC. In any case, in the coming weeks theorists will derive this operator using extra dimensions, little Higgs, fat Higgs, unparticles, supersymmetry, old newspapers, golf balls, and tires. Yet the most important question is whether the asymmetry is real, and we're dying to hear from CDF and Belle... Looking forward to Session 06 of ICHEP, maybe we'll know more then :-)

Dispensable Discoveries

2010-05-16T09:56:00.005+02:00

While around the world particle physicists are working frantically to produce important new results to be shown at ICHEP 2010, new discoveries get claimed in an asynchronous way. And some of them in a very asynchronous way, I should say, since they are based on 40-years-old data.

Old data may well still hold in custody surprising things that we have yet to unearth: in the course of the last 70 years physicists have produced collisions between hadrons in a number of ways, inside detectors of all kinds. And while of course we cannot ex post improve the detecting devices with which those data were collected, we have in our hands today more computing power in a 30-bucks cell-phone than we had thirty years ago in a large experiment.

The corollary is that it may make sense to look back at old data, in search of surprises. But there is a catch: if you are going to do that, you have better try using the knowledge we have picked up in the meantime: if you do not, your work cannot be taken very seriously. The other catch is that it may require you to copy by hand data which is only archived in printed form, as is the case I am going to describe here!

The Narrow Resonance at "About 755 MeV"

The Cornell arxiv is, as always, the source. In a preprint appeared there a few days ago prof. Mario Gaspero, from University of Rome "La Sapienza" (a place ripe with highly distinguished physicists) claims the observation of a narrow resonance in antiproton-neutron collisions produced in 1970! Let us have a closer look.

The announcement appears to be not new -the paper is a writeup of a talk given in November 2009. However, I think I can be excused for having overlooked a talk on hadron spectroscopy held in Tallahassee. Less excusable is having missed basically the same information when it first appeared one year ago, in a previous preprint. Never mind -I can be a monster with a thousand eyes when I play chess, much less so when I browse preprints.

But what is this all about, after all ? Okay, I will describe it in short, but first let me quote from the Abstract:

"A narrow peak in the pi+/pi- mass distribution was seen by the Rome-Syracuse Collaboration in pbar n -> 2 pi+ 3 pi- annihilation at rest in 1970. It was ignored for 40 years."

I think that this remark sets the stage. Letting go with qualitative, grudge-bearing sentences like "it was ignored for 40 years" in the abstract of a single-authored paper is, in my humble opinion, an indicium that something is not kosher. From now on, the mind of a bastard like me opens a parallel processor during the reading: one which constantly tries to find other "crackpot" hints.

The abstract continues as follows:

"The reanalysis of this peak finds that it has the mass 757.4 [...] MeV and a width consistent with the experimental resolution. The evidence of the peak is 5.2 standard deviations. [...]"

Okay, there was an analysis, so this is a re-analysis. This to me means that the signal was not "ignored for 40 years". But it is interesting, and so the paper deserves to be read.

The data comes from a collaboration which studied the decay of the omega into two pions, using the Brookhaven National Laboratory's 30-inch bubble chamber. According to the author,

"in 1970, the Rome-Syracuse Collaboration (RSC) [...] found an unexpected result: the pi+ pi- mass distribution of 1496 annihilations at rest [...] had a narrow peak at about 755 MeV. This distribution is shown in Fig.1a. A chi^2 fit of this distribution found that the peak had a significance of about 4.5 standard deviations and a width lower than the experimental resolution. No relation was found between the pi+ pi- and other angular and mass distributions. These facts suggested to the RSC that the peak was generated by a fluctuation."

For those of you who are already feeling they are in the dark, let me explain in simple terms what this is about. First of all, while today we are used to create millions of proton collisions per second at ultra-TeV energies to understand the structure of matter at the shortest distance scales, and employ detectors that are allegedly the most complex creations of mankind ever, forty years ago we used collisions at energies of few GeV -the equivalent of a few proton masses- and inspected each collision within the volume of a vessel filled with liquid, by taking pictures at the trails of bubbles left by the charged particles produced in the collisions.

As for the "chisquare fit" and the "4.5 standard deviations": this is experimental physicist's jargon, to say how a mass distribution such as the one below may be approximated by a smooth curve, and suggest the presence of a significant feature. 4.5 standard deviations occur by chance as a random fluctuation of the background only a few times every million trials. Such a rarity is usually sufficient to claim that the feature is real, but this, crucially, assumes that one knows extremely well how the background is distributed.

Finally, the width. A short-lived particle has a finite width, but if the detector resolution is not good enough, the resonance curve in the mass distribution will look like a Gaussian shape with sigma equal to the resolution. If, however, a bump is found with a width significantly smaller than the experimental resolution -as appears the original observation in our case-, this cannot in any way be a resonance.

Professor Gaspero however feels compelled to investigate the effect in more depth. He continues by explaining that only by the early 1990s the properties of baryon annihilations started to get understood better, such that a prediction for the mass distribution -the background!- could be drawn on top of the 1970 data.

This is shown in the figure on the left, with dashes (ignore the leftward dot-dashed "phase-space" distribution, which only shows how the data would distribute if QCD had nothing to do with the interaction). The histogram represents the experimental data: it seems that the three-bin bump at 755 MeV does not agree with the dashes... But it does not look like a 4.5 standard deviation thing! To get 4.5 standard deviations, you have to forget the background shape prediction, and perform a narrow-range polynomial plus Gaussian fit (the full curve). The dots show how the polynomial alone behaves.

Anyway, let us follow the paper:

"A year after the communication of the preliminary results of this analysis, the OBELIX collaboration presented the preliminary results of the analysis [...]"

OBELIX was an experiment which studied a quite similar reaction; one that should have produced the same new particle. An overlay of the RSC and OBELIX mass spectra normalized to each other is shown on the right. The author notes that

"At that time, the absence of the peak in the OBELIX data confirmed the opinion that the RSC peak was a fluctuation".

So is this a fluctuation, after all ? One experiment sees a bump in an otherwise allegedly smooth distribution, this appears significant but not overly so, and another experiment does not see anything. Where is the crackpot index going to point at, in the end ?

The verdict

In the end, I believe professor Gaspero is innocent of the charges we have piled up against him. While the article may be written in a slightly non-conventional way, and while his fits are all less than convincing, he does have a point. The reason is to be found in another paper, which he finally mentions:

"Recently, Troyan et al. claimed to have observed several narrow pi+ pi- peaks in the reaction np -> np pi+ pi- with neutrons of 5.2 GeV/c. One of these peaks has a mass close to 755 MeV. The coincidence of the mass of the RSC peak with that of Troyan et al. suggested to reanalyse the RSC data."

Indeed, by checking the reference, one finds a nice mass distribution, where Troyan et al. imaginatively fit no less than 10 new resonant states of charged pion pairs! It seems far-fetched to believe these are all honest-to-god resonances, but in truth the bump sitting at 755 MeV is one of the most significant ones -it does look like a hadronic particle of small width.

Above is the relevant figure. Admittedly, the "freedom fit" (one would usually call such smooth curves interpolating perfectly all the data points a "french fit", but let us this time to pay homage rather than mock our french friends) is a bit imaginative... But who knows: there are a lot of mysteries in low-energy quantum chromodynamics!

Gaspero in his paper tries to convince us that the excess observed by RSC at 755 MeV is significant enough to be considered a new particle. He estimates the significance of the observation in a way which is rather objectionable, but I do not wish to criticize such small prints. The particle may be there, or it may not. The problem, to me, is another. I sense, or imagine thanks to my fervid imagination, that this paper is written to get revenge rather than to claim a discovery which, leave alone belated, is not even a "first observation" -since if the particle is there, Troyan and collaborators are to be credited. Revenge is a strong word -let's say to have the final word on a controversy with colleagues who did not believe the new particle 40 years ago. Am I right ? I do not have a clue, and I do not wish to sound disrespectful to my colleague in Rome, so I will stop my conjectures here.

The problem is that the existence of these low-energy hadronic bound states does little to further our understanding of strong interactions, at least as long as they remain just mass peaks of unknown properties. Alas, we cannot predict them nor calculate their properties with the existing theoretical tools. If you let me take a radical stand, we need more theoretical ideas here! Lattice QCD may come to the rescue, but I believe there still are rough edges to smoothen there.

To conclude, I do not know if I learned something meaningful today, but I enjoyed reading this paper. And I imagine that in 2050 somebody from the OBELIX collaboration will come up with an explanation of why they did not, in fact, see the 755 MeV resonance... Stay tuned, but get yourself a comfortable chair!

In and out

2010-05-13T12:21:00.010+02:00

Two months and a few weeks before the start of the show... and my first post on this blog. Where should I start ? Writer's block, already ? Uhuh, don't get me wrong, I have plenty to write upon - otherwise I would not be here, would I ?

ICHEP is not the first conference for which I belong to the local organising committee, but it is definitely the biggest one. A conference with several hundreds of people flocking from all over the world is clearly a challenge for the organisers. We knew it from the beginning, and thus we have prepared it since... 2008. A good thing to plan things ahead -- but it makes the conference look very far away in time.

A few blinks later, it is already mid-May 2010, and even though you have kept on filling in Powerpoint slides, Excel sheets and Indico timetables, you have not realised fully that the conference starts... hem, now, basically. Writing this blog will definitely help me to keep track of time. Well, this and a couple of other tasks related to the conference, in particular the organisation of the parallel sessions... which will certaily take me more and more time as the opening of the conference approaches.

As both a physicist and a conference organiser I plan to provide you with glimpses of on-stage news but also off-stage surprises. And I expect quite a few. Physicswise, in addition to the fact that LHC is taking data to be presented here, we expect many other new results from high-energy experiments -- as shown by the number of talks proposed for parallel sessions. And as far as organisation is concerned... well, France is a country full of surprise, as I learned when I organised an earlier conference near Paris -- but I will keep this story for another post...

In any case, welcome to this blog, and welcome Paris for this new edition of ICHEP. We will do our best to make it a truly memorable conference for you -- as it surely will be for us !