Archive for December 18, 2009

An Excellent Start for LHC Physics (Seminar II)

Today the second public status report on the LHC was held. The presentations from the various experiments are on this public web page, and a recording of the session will be available from the CERN Document Server.

Fabiola Gianotti

Fabiola Gianotti, spokeswoman of ATLAS, delivering the ATLAS report. Several famous physicists can be spotted among the audience.

The LHC has delivered a useful data sample for pp collisions at a center-of-mass energy of 900 GeV (roughly 20 μb-1). There was also a very small sample at 2360 GeV (order of 1 μb-1), which exceeds the highest energy of the Tevatron at Fermilab.

The experiments used these data to establish the performance of essentially all subdetector systems, and to demonstrate the reconstruction of physics benchmarks. Many of these benchmarks are common, so one can make a direct comparison. Here are the reported widths of reconstructed signals, in MeV:

collaboration Ks→ππ Λ→pπ φ→KK π0→γγ η→γγ
ALICE 5.2 1.9 5.3 7.2
ATLAS 8.2 3.2 19
CMS 7.6 3.1 4.6 16 53
LHCb 4.3 1.4 11


The collaborations their capabilities to identify charged particle species. The plots from ALICE are particularly impressive, showing very clean dE/dX separation in their TPC and ITC, augmented by e/π separation in the TRD and nice separtion by TOF. They are planning a publication on the multiplicities and momentum spectra of pions, kaons and protons. ATLAS and CMS also shows dE/dX separation of e/π/K/p, and CMS used this in constructing their φ→KK signal. ATLAS also has a TDR with a very nice separation. LHCb demonstrated that their RICH detector can identify charged kaons well.

All detectors are able to reconstructed leptons (electrons and muons) well. ATLAS, CMS and LHCb showed di-muon candidates, and those from CMS and LHCb are potential J/ψ signals. It will be interesting to see these samples grow from a handful of events to clear signals, hopefully with a clear prompt and non-prompt component from B hadron decays (at higher energies).

Nice examples of di-jet and tri-jet events were shown by ATLAS and CMS, and both collaborations were able to show a jet pT spectrum, with an excellent reproduction by the simulation. More importantly, the missing transverse energy (MET) distributions were shown and the canonical relation between MET and ΣET, again reproduced very well by the simulation. The CMS Collaboration showed results from their _Particle Flow_ algorithm, with a very good calibration and even the reconstruction of neutral hadrons in the calorimeter.

There were two presentations by the forward experiments, TOTEM and LHCf, showing that their apparatus is working and they are able to understand the signals to some degree.

The most exciting note was struck by the ALICE and CMS Collaborations, who gave a glimpse of their fast-track physics analyses. ALICE has already submitted a publication to EPJ on the measurement of the charged particle pseudorapidity (η) density (arXiv:0911.5430). They will also measure the anti-proton/proton ratio, which is sensitive to the physics of hadronization, and the pT spectra of pions, kaons and protons. The CMS Collaboration will update the ALICE measurement of dN/dη, covering a wider range in η with much higher precision.

Jim Virdee

Jim Virdee, spokesman of CMS, delivering the CMS report. People were standing in the back and sitting in the aisles.

Clearly a tremendous amount of rapid work has been done by all collaborations, and the very brief presentations today showed only a very limited set of highlights. It is amazing how well the reconstruction and analysis of the data has proceeded already, and how well the simulations reproduce the data, especially at the level of the detector simulation. Of course this is only the beginning, but I can confidently predict and explosion of physics results to be published next year!


December 18, 2009 at 11:07 am 1 comment

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