Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

CHAPTER 2: The CMS experiment at the LHC 19The results of analyzing the 36.1 pb −1 of the proton-proton collision data in theframework of this thesis are given in detail in Section 5.6.Currently the LHC machine is operating at a center of mass energy of 7 TeV and theCMS experiment has already been able to record an integrated luminosity of protoncollisions equivalent to more than 2000 pb −1 = 2 fb −1 in 2011, hence 55 times moredata than what had been collected in 2010. The LHC has been designed to operateat an even higher center of mass energy, being two times larger than the one achieved.It has been decided [36, 37] that the machine continues running at a center of massenergy of 7 TeV at least until the end of 2011.2.1.2 Particle Detectors at the LHCFour main experiments detect collisions which occur at the LHC. Two of them, beingCMS (Compact Muon Solenoid) [38] and ATLAS (A Toroidal LHC ApparatuS) [39],are general-purpose detectors. They are designed to address a wide variety of physicsquestions. Searching for the Higgs particle, exploring physics at TeV scale and lookingfor evidence of physics beyond the Standard Model such as supersymmetry are someof the goals which are followed by the multi-purpose experiments of the LHC.The other two, ALICE (A Large Ion Collider Experiment) [40] and LHCb (LargeHadron Collider beauty) [41], are special-purpose detectors. While the former is dedicatedto heavy ion collisions, the latter is specialized to the studies of b quark physics.The location of these main experiments around the main ring of the LHC is shown inFigure 2.3.In addition to the four main experiments, the LHC hosts two more detectors. Afirst one, which is named LHCf (Large Hadron Collider forward) [42], is intended tostudy the particles that are produced in the forward region of the interaction pointof proton collisions. It consists of two pieces which are installed on either side of theATLAS detector, 140 m away from the interaction point. A second one, which is namedTOTEM (TOTal Elastic and diffractive cross section Measurement) [43], is designedto measure the total proton-proton cross section. It is located on the forward regionof the CMS detector.2.2 The CMS ExperimentDetectors are constructed in order to observe and analyze the collisions which are providedby the collider. Exploring the vast physics opportunities in the proton-protoncollisions at high energy scales reachable at the LHC, requires to build general-purposeparticle detectors such as the CMS experiment. Particle detectors are generally designedto be symmetric in space to make the physics analysis easier. The particle isdetected when it interacts with matter. According to the various kind of particle interactions,the detectors contain appropriate materials, each of which is aimed to identifysome special type of particles or to measure particular properties of the particles. Thesub-systems are arranged in such a way which yields the detector to have an onion-likestructure. A slice of the transverse view of the CMS detector is shown in Figure 2.5.