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

90 CHAPTER 5: Estimating of the Jet Energy Scale Calibration Factorfrom the decay of the top quark in the topology t → Wb → q¯qb. For a typical event,the distribution of the P KinFit (∆E l , ∆E b ) is shown in Figure 5.12.P KinFit(∆E l,∆E b)10.80.60.40.21.4 01.31.21.1∆E b10.90.80.70.60.6∆E l0.7 0.8 0.9 1 1.1 1.21.3 1.4Figure 5.12: The distribution of the P KinFit (∆E l , ∆E b ) for an e+jets t¯t event.The central point of the distribution of the P KinFit (∆E l , ∆E b ), which is expressedby P KinFit (∆E l = 1, ∆E b = 1), corresponds to the situation when no correction isapplied. It is understood from Figure 5.12 that the maximum of the fit, for thatparticular event, does not happen at the center of the grid, which shows the need forthe residual jet energy corrections. In order to justify the need for the residual jetenergy corrections, one has to increase the statistics and may look at the distributionof the values of the P KinFit (∆E l = 1, ∆E b = 1), which is shown in Figure 5.13.There is a clear peak at zero which can be partially interpreted as follows. Thehypothesis that the three jets in e+jets t¯t event originate from the top quark is notfulfilled and the imposed mass constraints are not converged, when no correction isapplied. This observation justify the nead for the residual jet energy correction. Alsothe events, where the MVA method is not able to return the correct jet-parton combination,contribute to the peak at zero as will be discussed in what follows.For a given event, the point with the maximum probability in the distribution of theP KinFit (∆E l , ∆E b ), which is expressed by PKinFit max (∆E l, ∆E b ), shows the point wherethe imposed constraints are maximally fulfilled. The distribution of the PKinFit max (∆E l, ∆E b )value for all events that already pass the radiation veto cut, is shown in Figure 5.14.It is seen from Figure 5.14 that for most of the events, a maximum probabilityexceeding 0.9 is returned by the kinematic fit method, representing those events forwhich the maximum happens somewhere in the region that is scanned. Some of theevents are peaked around zero which represent those events where no maximum is foundin the window of ±40% around the measured values of the jet energies or events thatthe fit is not converged for the given jet-parton combination. This can be understoodby looking at the two-dimensional histogram which is made by plotting the distribution