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

Chapter 5Estimating of the Jet Energy ScaleCalibration FactorIn the previous chapter, the selection cuts, which are applied to enhance the purity ofthe e+jets signal events in the final selected sample, were introduced. At the end whenthe survived event sample is enriched in e+jets t¯t events, one can try to reconstructthe W boson and the top quark four-vectors by combining the individual objects andreconstruct backwards the decay chain. We divide the t¯t system into two separateparts depending on the decay products of the W boson, namely hadronic and leptonic,t h → W h b h → q¯qb h ,t l → W l b l → eν e b l ,where indexes “h” and “l” refer to the hadronic and the leptonic parts, respectively.Since the four-vector of ν e is not reconstructed, the four-vectors of the leptonic Wboson and subsequently the leptonic top quark can not be made from their correspondingdecay products. While the leptonic part would remain non-reconstructed,the hadronic part can be fully reconstructed. In the hadronic part, the light quarks qand ¯q, hadronize and produce the light jets. Given that the light jets are known amongthe collection of the reconstructed jets, the corresponding four-vectors are summedand make the four-vector of the hadronic W boson. Hence in order to reconstruct thehadronic W boson, one needs to determine which jets arise from the W boson decay.Consequently, in order to reconstruct the hadronic top quark, one has to determinewhich jet is the hadronic b jet. Although the leptonic top quark would not be fullyreconstructed, in order to benefit from the kinematics of the leptonic b quark, onemay be interested in associating a jet to the leptonic b quark, as well. The task ofassigning reconstructed jets to partons arising from t¯t → eν e q¯qb¯b, is a key topic whichis explained in more detail in Section 5.1.Having associated the jets to the partons, one can measure the invariant masses ofthe hadronic W boson and the hadronic top quark. In Section 5.1.6, it is shown thatthe measured invariant masses of the W boson as well as of the top quark, deviatefrom their nominal values that are used in the generation step of the simulation. Thisobservation motivates to correct the energy of the jets so that the measured masses ofboth the W boson and the top quark are peaked at their generated values. Applying67