My Bet ? A Fourth Generation!

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!

The CMS Momentum Scale And Resolution

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!

A Spectroscopist's Delight!

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!

Electroweak Signals From CMS

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!

Upsilons Popping Up In CMS!

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.

ATLAS Reach Predictions for a SM Higgs

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.

Prayer to the Funding Agency Reviewer

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.

Rumors About A Light Higgs

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

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...

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

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!

News From The Third World Of Research - Italy

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.

And CMS, in the meantime...

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!

Dispensable Discoveries

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 an exciting company!

This is my first post in the ICHEP 2010 blog, and sort of a dry run for the whole construct from my perspective. This post -as well as others with little physics content- will be published only here; posts with meaningful physics information will also be re-blogged on my personal web site, a quantum diaries survivor. However, here you will only find thoughts and comments relevant to the topics covered by ICHEP -and judging by the program, our hands are full already!

So here I first of all wish to thank Marco Cirelli and the other organizers for realizing the value that physics blogs offer to the community.

And while I am at this, let me say that I have recently advocated a more official support of institutional web sites for the job of scientific blogs, and for their positive role for physics outreach and popularization of science. I did this at a conference, Comunicare Fisica 2010, held in Frascati one month ago. There, my ideas did not receive a lot of positive feedback. Nonetheless, being invited to be part of a free blogging effort under the sponsorship of an institution such as ICHEP is, in my opinion, worth much more than a thousand empty words -and tens of thousands had been spent on this subject after my suggestion in Frascati!

If you are interested, you may find more discussions on the above topic in my blog, as well as in the blog of Marco Delmastro -who is also going to contribute here- and Peppe Liberti. The latter two, however, only in Italian.

So I am exciting to see how this new endeavour develops! Stay tuned!