Luminosity is not the whole story

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!

What should we expect from LHC?

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):

1. 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.
2. 100-1000 μb^-1: any hint of a $J/\psi \to \mu \mu$ peak should now be clearly visible;
3. 1-10 nb^-1: more jets. And of course more jets-related measurements.
4. 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.
5. 100-1000 nb^-1: more and more jets. The first inclusive muon measurements should be feasible. Signal from prompt photons should have been isolated.
6. 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.
7. 10-100 pb^-1: first B-physics related measurements. Something could already be said about some exotic scenarios, and some SUSY points.
8. 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.