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