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

CHAPTER 2: The CMS experiment at the LHC 21Figure 2.4: A typical electromagnetic shower which is initiated by a photon and isdevelopping by bremstrahlung and pair production processes.interaction of hadrons with the heavy nuclei of the material used in the HCAL subsystem.Analogous to X 0 , the typical length scale parametrizing the thickness of thehadronic calorimeter is the nuclear interaction length λ which is defined as the meanfree path length of the hadron before interacting with the material.At the outermost part of the CMS detector, the muon system is used which is designedto detect and identify muons. Due to the rather large mean free path, muons are onlyinteracting weakly with the calorimeter material and deposit a small amount of theirenergy in the calorimeter system and penetrate the last layer of the detector, which isaimed to detect muons. Since one of the important discovery channels of the StandardModel Higgs particle is via pp → H → ZZ ∗ → µµµµ, a sub-detector with a high resolutionhas been dedicated for muon identification and measurements. The importanceof this has affected the naming of the detector.Between the calorimetry and muon system, there is the solenoid magnet which generatesa powerful magnetic field of 3.8 T. The magnetic field is confined using the ironyokes implemented outside of the magnet coil in the muon system.The Compact Muon Solenoid experiment [44] is an apparatus with 21 m length,15 m width and 15 m height, having a total weight of about 12500 tonnes, most ofwhich is due to the iron yokes. Comparing with the other general-purpose detectorof the LHC, ATLAS with 46 m length, 25 m width and 25 m height, CMS is muchmore compact. The CMS apparatus is the only detector of the LHC experiment whichwas constructed on the surface in fifteen sections, before being lowered undergroundinto the cavern for assembly and installation. A schematic view of the Compact MuonSolenoid can be found in Figure 2.6.The four-momenta of the particles which emerge in the proton-proton interactionsare described using a spatial coordinate which is centered on the interaction point. They-axis points vertically upward while the x-axis is chosen to point to the center of theLHC. The z-axis is then selected along the beam direction in such a way which yieldsto obtain a righthanded coordinate. Based on this choice of cartesian coordinates,