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Scientific Report 2007-2009<br />
Particle physics<br />
P5. The CMS experiment at the CERN LHC<br />
The Large Hadron Collider at CERN, which started<br />
the operations in 2008-09, is the highest energy accelerator<br />
in the world and will be a unique tool for fundamental<br />
physics research for many years. The LHC will provide<br />
two proton beams, circulating in opposite directions, at<br />
an energy of 7 TeV each (centre-of-mass √ s = 14 TeV).<br />
The CMS experiment is a general purpose detector to<br />
explore physics at this unprecedented energy scale. It is<br />
expected that the data produced at the LHC will elucidate<br />
the electroweak symmetry breaking, for which the<br />
Higgs mechanism is presumed to be responsible, and provide<br />
evidence of physics beyond the standard model such<br />
as supersymmetry or other unknown mechanisms. CMS<br />
will also be an instrument to perform precision measurements,<br />
e.g., of parameters of the Standard Model, mainly<br />
as a result of the very high event rates: the LHC will be<br />
a Z factory, a W factory, a b quark factory, a top quark<br />
factory and even a Higgs or SUSY factory if these new<br />
particles have TeV scale masses.<br />
The CMS (Compact Muon Solenoid) detector, shown<br />
in Fig. 1, measures roughly 22 meters in length, 15 meters<br />
in diameter, and 12,500 metric tons in weight. Its<br />
central feature is a huge superconducting solenoid, 13<br />
meters in length, and 6 meters in diameter. Its compact<br />
design is large enough to contain the electromagnetic and<br />
hadron calorimetry surrounding a silicon tracking system,<br />
and its high field (4 Tesla) allows a superb tracker<br />
detection system. Muon momenta are measured by gas<br />
chambers in the iron return yoke.<br />
long been understood that H → γγ can be detected as<br />
a narrow mass peak above a large background and then<br />
the resolution of the calorimeter is crucial. This led to<br />
the choice of high density scintillating PbWO 4 crystals,<br />
providing a compact, dense, fast and radiation resistant<br />
material with a resolution better than 0.5 % for high energy<br />
photons. Details of Rome contribution to the CMS<br />
calorimeter are given in ”The Lead Tungstate Crystal<br />
Calorimeter of the CMS experiment” on this Report.<br />
The H → γγ predicted signal is shown in Fig. 2.<br />
Figure 2: H → γγ signal simulated in the CMS experiment<br />
for a Higgs mass of 130 GeV/c 2<br />
Besides the Higgs search, our main interests in the<br />
physics analyses are the search of supersymmetric<br />
particles through electron and photon decays and of<br />
new, heavy Z bosons through their electron decays.<br />
References<br />
1. G. L. Bayatian, et al., J. of Phys. G: Nucl. Part. Phys.<br />
34, 995 (2007).<br />
2. S. Chatrchyan, et al, JINST 3, S08004 (2008).<br />
3. P. Adzic, et al., JINST 2, P04004 (2007).<br />
Figure 1: Schematic view of the CMS detector<br />
Our group mainly contributed to the project and construction<br />
of the electromagnetic calorimeter, which is designed<br />
on the benchmark Higgs boson decay into two<br />
photons. The H → γγ channel is crucial for the discovery<br />
of Higgs particles at masses beyond the upper reach<br />
of LEP (114 GeV/c 2 ) and below about 150 GeV/c 2 . The<br />
challenge for discovery of a Higgs in this mode is the<br />
small branching fraction of about 0.002. The γγ decay<br />
mode can be well identified experimentally but the signal<br />
rate is small compared to the background. It has<br />
Authors<br />
L.M. Barone, F. Cavallari 1 , D. del Re, I. Dafinei 1 , M.<br />
Diemoz 1 , E. Di Marco, D. Franci, E. Longo, G. Organtini,<br />
A. Palma, F. Pandolfi, R. Paramatti 1 , S. Rahatlou, C.<br />
Rovelli 1 .<br />
http://www.roma1.infn.it/exp/cms/<br />
<strong>Sapienza</strong> Università di Roma 112 Dipartimento di Fisica