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

92 CHAPTER 5: Estimating of the Jet Energy Scale Calibration Factorof P KinFit (∆E l = 1, ∆E b = 1) versus P maxKinFit (∆E l, ∆E b ) as shown in Figure 5.15.P KinFit (∆E l =1,∆E b =1)10.80.60.40.200 0.2 0.4 0.6 max0.8 (∆E l,∆E b)1P KinFitFigure 5.15: The distribution of P KinFit (∆E l = 1, ∆E b = 1) versus P maxKinFit (∆E l, ∆E b ).Only e+jets t¯t events survived the radiation veto cut, are taken into account.It is clear from Figure 5.15 that for the events with a maximum fit probabilitybelow 0.9, the probability returned by the fit method when no correction is applied,is populated around zero. This means that the jets are assigned wrongly to the hardscatter partons by the MVA method or the event is not correctly reconstructed due tothe radiations.Each selected event is required to have a PKinFit max exceeding 0.98. This cut aims to keeponly those events for which the hypothesis is correct and the maximum occurs somewherein the space which is being looked at around the uncorrected energies of the jets.No cut is applied on the P KinFit (∆E l = 1, ∆E b = 1), since cutting on this parameteryields to reject those events where a maximum is found in the window of the residualcorrection and they have to be used for the final estimation of the jet corrections. Inaddition to the radiation veto cut and cutting on the PKinFit max (∆E l, ∆E b ) variable, athird one which is the last in the sequence of extra event selection cuts, is introduced.There are some events for which the fit procedure does not converge in some points inthe space of P KinFit (∆E l , ∆E b ). It is desired to have a fit probability which is smoothlydistributed. Therefore, the events which contain at least one point in the grid wherethe kinematic fit does not converge, do not contribute in the rest of analysis.The additional analysis specific cuts, which are applied on top of the basic event selectioncriteria as already introduced in Table 4.6, are summarized in Table 5.3 andthe number of events passing each step are also mentioned individually for signal andrelevant backgrounds.It can be understood from Table 5.3 that the t¯t → other events constitute thedominant background to e+jets t¯t signal events. Since the simulated sample of theQCD multijet events are statistically limited, the uncertainty on the 9.4 events is largecorresponding to about 8.6 events. Therefore, the t¯t → other is still the dominantbackground to the analysis.