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More on the Z lineshape at LHC December 19, 2008

Posted by dorigo in personal, physics, science.
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Yesterday I posted a nice-looking graph without abounding in explanations on how I determined it. Let me fill that gap here today.

A short introduction

Z bosons will be produced copiously at the LHC in proton-proton collisions. What happens is that a quark from one proton hits an antiquark of the same flavour in the other proton, and the pair annihilates, producing the Z. This is a weak interaction: a relatively rare process, because weak interactions are much less frequent than strong interactions. Quarks carry colour charge as well as weak hypercharge, and most of the times when they hit each other what “reacts” is their colour, not their hypercharge. Similarly, when you meet  John at the coffee machine you discuss football more often than chinese checkers: in particle physics terms, that is because your football coupling with John is stronger than your chinese-checkers coupling.


Result now, explanation later December 18, 2008

Posted by dorigo in personal, physics, science.
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Tonight I feel accomplished, since I have completed a crucial update of the cornerstone of the algorithm which provides the calibration of the CMS momentum scale. I have no time to discuss the details tonight, but I will share with you the final result of a complicated multi-part calculation (at least, for my mediocre standards): the probability distribution function of measuring the Z boson mass at a certain value M, using the quadrimomenta of two muon tracks which correspond to an estimated mass resolution \sigma_M, when the rapidity of the Z boson is Y_Z.

The above might -and should, if you are not a HEP physicist- sound rather meaningless, but the family of two-dimensional functions P(M,\sigma_M)_Y is needed for a precise calibration of the CMS tracker. They can be derived by convoluting the production cross-section of Z bosons \sigma_M at a given rapidity Y with the proton’s parton distribution functions using a factorization integral, and then convoluting the resulting functions with a smearing Gaussian distribution of width \sigma_M.

Still confused ? No worry. Today I will only show one sample result – the probability distribution as a function of M and \sigma_M for Z bosons produced at a rapidity 2.8< |Y| <2.9, and tomorrow I will explain in simple terms how I obtained that curve and the other 39 I have extracted today.

In the three-dimensional graph above, one axis has the reconstructed mass of muon pairs M (from 71 to 111 GeV), the other has the expected mass resolution \sigma_M (from 0 to 10 GeV). The height of the function is the probability of observing the mass value M, if the expected resolution is \sigma_M. On top of the graph one also sees in colors the curves of equal probability displayed on a projected plane. It will not escape to the keen eye that the function is asymmetric in mass around its peak: that is entirely the effect of the parton distribution functions…

Hectic week December 4, 2008

Posted by dorigo in personal, physics, science.
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The regulars here will have already noticed by now that my posting rate has fallen this week. I have been busy with three different physics analyses, trying to make some progress in each.

The first project is the calibration of the momentum scale in CMS. I have discussed the issue elsewhere a couple of times; I am slowly converging to an understanding of how to treat the Z boson lineshape -which receives contributions from a number of different sources and effects: parton distribution functions in the projectiles, electromagnetic and weak radiation effects, interaction of the final state products of Z decay with the material of the tracker. All this must be dealt with in a coherent fashion to extract the most information possible from the Z decays we will reconstruct in CMS. We have a small but focused group working at the momentum scale calibration, including worthy physicists from Torino University, plus Marco and me. This week, I have tried to determine the effect of parton distribution functions alone, to insert it in our algorithm, but something still escapes me, and I want to do things as well as I can -which sometimes take little extra effort from a mediocre result, but in this case seems to be requiring a lot more care.

The second is the search for Higgs boson decays in the final state arising when H decays to two Z bosons, and one of the Z decays to a lepton pair, while the other decays to a pair of jets. Usually this final state, which is very hard to exploit at low Higgs masses due to the large backgrounds, is used for high-mass searches only (above 200 GeV). We want to extend it to lower masses, where the Higgs is more likely to be, using the Z \to b \bar b decay, which Mia and I have a lot of experience in detecting in hadronic environments. Mia will present some results of this study tomorrow at CERN, so we have been working at this heavily this week.

The third topic is the evaluation of the chances of CMS to detect a similar signature of multi-muon events that CDF has seen in its data. The CDF signal is probably just a not well understood background, but it makes sense to size up the capability of CMS to detect a similar signature with early data. This requires understanding muon sources without using real data, and it is a bit far-fetched, but it is perfectly sound as a masters’ thesis topic, one on which Franco and I in fact have a student working. I have not worked much on this topic this week, but it still has absorbed a little of CPU.

I have a thick agenda of pending things to do, which has grown longer in the last few days. One thing is to post more commentaries on the multi-muon analysis by CDF here. Another is to progress with a document I am writing. A third is to review a 40-pages long CDF paper draft for the Spokespersons Reading Group, to which I proudly belong. A fourth is to organize the upcoming meeting of the CMS-Padova software-analysis group, which will convene in ten days. A fifth is to prepare my next trip to CERN, which will be from next Monday to next Friday. I do hope that I will be able to post more in the next few days… if I survive.


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