Atlas vs CMS: 6-0 (and we haven’t even started playing) May 19, 2007Posted by dorigo in humor, news, personal, physics, politics, science.
Yesterday I had a hearty laugh at a seminar – I should learn how to control myself better, because I think my attitude manages to piss off everybody at times. But there just are instances when I cannot restrain myself. It is just as if the comicity of something that everybody else finds utterly normal is capable of striking me with double force.
The seminar was given by Marcela Carena (right), a distinguished Fermilab theorist, in the CDF “theatre”, the largest meeting room we have in our detector building. Her talk drew full attendance. She had been invited to talk about “B physics, direct dark matter detection, and supersymmetric higgs searches at colliders“.
Marcela’s talk was interesting and enlightening as always, and it is totally not her fault if I burst out laughing in the middle of the meeting (sorry, Marcela). The fact is, she showed a slide with projected exclusion regions on the MA-tan(beta) plane (two variables which are very useful to describe a subset of the parameter space of minimal supersymmetric extensions of the Standard Model [MSSM] theories) by Atlas and CMS. She had computed the points of the parameter space which could be excluded with a given amount of data (I believe it was 30 inverse femtobarns, the full monty of collisions the LHC may provide in a few years) by the two experiments performing searches for the decay of higgs bosons to pair of photons or to pairs of tau leptons. And the results were ridiculous!
Again, she has no fault – as far as I can tell her calculation is correct, and the ridicule is totally due to the volatility of projections that the two experiments manage to make in these days of anxious wait for the first LHC collisions. The two detectors are essentially very similar – they are both multi-purpose, they both have excellent calorimetry and tracking, they provide almost hermetic coverage to muons… Wouldn’t you expect that they perform to a comparable level ? I do. So, whenever I see plots describing the “discovery reach” or the “exclusion reach” of CMS and Atlas on a particular new physics signature, I tend to frown at large discrepancies, and attribute them to the different care with which those projections have been cooked up.
Now take a look at the two plots below.
These plots are quite busy, so before you can get the punchline you need some instructions. In the plane MA:tan(beta) you see a hatched black line which shows the region of the parameter space allowed by B-physics measurements (Bs decays to muon pairs, B decays to tau-nu, Bs oscillations, etcetera), for a particular choice of some additional parameters defining the details of the MSSM being considered (the so-called “Mh max scenario”, where SUSY particles have a mass of 1 TeV and the lightest neutral Higgs can assume its largest possible value). The yellow shading shows the part of the plane which the experiment can exclude with a search of higgs decays to gamma pairs, while the hatched green shading shows the exclusion possible with a search for vector boson fusion qq-> qqh followed by the decay of the higgs into tau lepton pairs. The red curve and region in the upper left corner are present-day excluded regions by CDF and D0, and they need not concern us further here.
Now, what is the message of these two plots ? There are several, and one of them triggered my laughs.
1) Atlas will be able to conclusively exclude the full parameter space (for the given choice of additional parameters) of MSSM with its searches of gamma-gamma and tau-tau signatures.
2) CMS does about as well as Atlas in the gamma-gamma search
3) CMS does “somewhat” worse with tau lepton pairs.
I think I would agree with you, nothing really hilarious. But these two plots, presented side by side, are just too much to digest: it is as if one told you: yes, we have two very similar detectors, with two groups of 2000-something similarly qualified physicists analyzing the respective datasets. And one detector will be irrelevant, while the other will wipe SUSY off the map!
Of course, the problem has to be with the prediction made for the higgs to tau pairs search in CMS. I will ask Simone Gennai, a colleague in CMS who has spent the better part of the last six years on tau detection in CMS, about these projections. Maybe he will be able to explain to me why is it that Atlas is expected to beat the s*** out of us in this particular search [something which I do not believe for a minute].
More in general, I cannot help feeling that these plots are dangerous. I recently gave a talk to the committee distributing funds to the italian particle physics experiments, the National Scientific Committee I of INFN, and I know that the committee would not have laughed much if some CMS colleague had presented those two plots side by side. Were the plots taken seriously, they would convey the message that one detector is enough in LHC… While the INFN is financing both.