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

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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.
CHAPTER 5: Estimating of the Jet Energy Scale Calibration Factor 93t¯t t¯t W+jets Z+jets t QCD→ e + jets → other → l + jets → l + l − + jets → l + jets multijetsSelection Table 4.6 307.8 39.1 30.2 4.5 13.5 35.6Radiation Veto 180.4 22.5 21.4 2.5 9.9 27.4PKinFit max75.2 6.3 3.4 0.3 2.5 9.4Gap Veto 66.8 5.5 2.8 0.3 2.2 9.4Table 5.3: Table of extra event selection cuts. The numbers are equivalent to anintegrated luminosity of 100pb −1 .Having applied the extra event selection cuts given in Table 5.3, one could reach a signalover background ratio of about 3.3 which yields an even more purified final sample ofselected events compared to the baseline event selection cuts. It is worth to note that ina fraction of selected events, surviving the baseline cuts, which corresponds to 17%, theMVA method is able to return the correct jet-parton combination while after applyingthe additional event selection cuts, this fraction is enhanced to 52%. This means thatthe extra event selection cuts are able to reject the badly reconstructed events yieldsto suppress the combinatorial backgrounds.Those events which are survived the additional selection requirements, are acceptedfor the final analysis.5.4 Extracting Jet Energy Scale Calibration FactorsThe estimation of the residual jet energy calibration factors is based on the outputof the kinematic fit. For each selected event, the kinematic fit technique returns atwo-dimensional fit probability P KinFit (∆E l , ∆E b ) in the dimensions of the residuallight and b jet energy corrections, as explained in the previous section. The jet energycorrection factors are then determinded by maximizing the fit probabilty returned bythe fit procedure as described below.The kinematic fit method is applied on the three reconstructed jets in e+jets t¯t event.The jets, which were associated to the three quarks originating from the hadronicbranch of the top quark decay t → Wb → q¯qb by means of the MVA method, areforced to fulfill the mass constraints imposed on the system. The four-vectors of thethree reconstructed jets, which are fed to the fit package, are parametrized using theTFitParticleEtThetaPhi parametrization for which the ratio of the energy over momentumE is kept constant and equal to the measured value during the fit procedure,|p|as already explained in Section 5.2.1. The parameters of the fit are chosen as follows.The parameter ǫ S is set to be 5 × 10 −5 . Also it is chosen to have ǫ F = 10 −4 . Theiteration procedure is repeated 30 times. If the fit is not converged after 30 times, thefitting is stopped and a value equal to zero is assigned to the probability of the fit for
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Vrije Universiteit BrusselFaculteit
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IntroductionThe Standard Model of p
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Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

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