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

CHAPTER 2: The CMS experiment at the LHC 23the spherical coordinates can also be introduced which are commonly used at hadroncolliders. In addition to the polar angle θ and azimuthal angle φ, a new variable is ofhigh importance at hadron colliders, namely the pseudorapidity η defined asη ≡ 1 ( E +2 ln pz) ( θ= −ln tan ,E − p z 2)where the equation is true in the massless limit of the particle with an energy of Eand a z component of the momentum vector of p z . Therefore, in order to describe theangular coordinates of the particle, it is common to use (η, φ) parameters instead of(θ, φ) variables.In the following sections the sub-detectors of the CMS experiment are briefly explained.2.2.1 Tracker SystemThe tracker system [45, 46] of the CMS detector is made up of silicon based detectorsto provide a high resolution detection required by the huge flux of charged particlesproduced in the high luminosity regime of the machine. The overall structure of thetracking system comprises a central cylindrical part, called tracker barrel, and twoseparate discs placed at either side of the barrel, called tracker endcaps. In total 13layers in the central region and 14 layers in the endcaps form the CMS silicon tracker.They overall exhibit a diameter of 2.4 m and a length of 5.4 m resulting in a trackingcoverage of |η| ≤ 2.5. Closest to the interaction point is a silicon pixel detector,comprises the innermost 3 layers in the barrel and 2 layers in the endcaps. A totalof 66 million pixels with a size of 100 × 150 µm 2 cover a distance of 4 to 11 cm fromthe beam line on a cylindrical barrel and endcaps structure with total length of 92cm. Besides the silicon pixel detectors, silicon strip detectors contribute in the trackersystem with a variable size from 10 cm × 150 µm to 25 cm × 150 µm. There are 9.6million strip channels in total. A longitudinal view of the tracker system can be foundin Figure 2.7, where only the silicon strip layers surrounding the pixel detector areshown.The transverse momentum of a charged particle with an energy of about 100 GeVproduced in the barrel part of the detector, is measured by the tracker system with aprecision of about (1-2)%.2.2.2 Calorimetry SystemECALThe electromagnetic calorimeter (ECAL) [47, 48] is aimed to measure the energy ofelectrons and photons. Similar to the tracker system, the ECAL is also charactrized bythe electromagnetic barrel (EB) and the electromagnetic endcap (EE) structure. Leadtungstate PbWO 4 crystals are used to construct the ECAL due to its high density andradiation hardness. It has a small radiation length of X 0 = 0.89 cm and a Moliere radiusof R M = 2.2 cm. Each barrel crystal covers an area of ∆η ×∆φ = 0.0174 ×0.0174with a length of 23 cm which corresponds to 25.8 X 0 . The arrangement of the barrel