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Atlas vs CMS: 6-0 (and we haven’t even started playing) May 19, 2007

Posted 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.



1. Tripitaka - May 20, 2007

I know nothing of the technicalities but those two slides side by side are perfect! It wouldn’t have been so funny if they gave CMS no SUSY space at all, but the piss-weak little strip they allocated has such comic emphasis, thats a classic!

2. franco - May 20, 2007


you are one case of PTLS – not post tubal litigation,
but post traumatic logorroic syndrome

3. Andrea Giammanco - May 21, 2007

Let me guess:
1- one of the studies is in fast simulation(*), the other with a full simulation
2- different systematics were taken into account.

(*)by the way, I was recently told that the (in)famous plot on Tevatron prospects for Higgs discovery, done some years ago, that you show quite often, was produced with PGS. Is it true? In this case, well, I can’t understand how is it possible that it was taken so seriously at the time…

4. dorigo - May 21, 2007

Tripitaka, you got it – that is exactly the point that made me roll over.

Franco, I yam what I yam. I love to write, and I do my little bit to do some physics outreach; plus, I appreciate feedback, and blogging has earned me a few friends (and some enemies). I hope you don’t mind.

Andrea, I am not sure whether Atlas did only a fast simulation study. Besides, I am much more surprised by the CMS result than by the Atlas one – it is quite unbelievable that CMS can only exclude that tiny region with the H->tau tau VBF mode. Anyway, the 2000 SUSY-Higgs workshop used SHW, a fast simulation of the “average detector” – a mid-point between D0 and CDF. But the study was actually of high quality, as far as I can tell (ok, I was part of the group). The 2003 study instead used real detector simulations, real data when possible, and found quite similar results… Yes, it should be taken seriously.

Cheers to all,

5. Andrea Giammanco - May 21, 2007

My experience for the TDR is that the collaboration agreed on quite conservative assessments on systematics, expecially for what concerns the jet energy scale.
(Well, at least I hope they will turn out to be conservative.)
An example from a particular analysis that I know very well: with roughly the same efficiency and the same purity (coming from more or less the same cuts), you have 12% ATLAS and 36% CMS. Reason: different assumptions of the jet energy scale uncertainty after a certain integrated luminosity. It IS, indeed, motivated to have different assumptions, since the CMS hadron calorimeter is the weakest part of the experiment(*) while the ATLAS calorimeter is very good; but still, I suspect (and hope) that we played the overconservative.

(*) and in fact we expect to have a very good jet energy scale determination when using Particle Flow, but a complete PF is not ready yet, so we can’t seriously estimate what will be this uncertainy after X fb-1.
But the preliminary plots that people are starting to show (including one from a student of my institute) make me quite optimistic: we will really gain a lot, and physics with jets (or tau-jets) in the final state will not be an handicap for CMS (which is instead the message coming from the TDR, where only the purely calorimetric definition of jet was used…)

6. dorigo - May 23, 2007

Hi Andrea,

it might indeed be that the rather bleak assessment of potential of H->tau tau searches in CMS is due to a too conservative approach. Probably Atlas decided to make some extrapolations while CMS relied of what they could show.

The difference of the two plots above may well be due to the underlying assumptions… But it still looks comical to me.


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