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

80 CHAPTER 5: Estimating of the Jet Energy Scale Calibration FactorPurity10.90.80.70.60.50.40.30.20.10Likelihood0.2 0.4 0.6 0.8 1EfficiencyFigure 5.7: The purity versus efficiencyplot when cutting on the MVA output forthe selected e+jets t¯t events with a correctjet-parton combination.Figure 5.8: The purity versus efficiencyplot when cutting on the MVA output forall selected e+jets t¯t events.Also shown in Figure 5.8 is the distribution of the purity versus efficiency when cuttingon the Likelihood response which is plotted for all selected e+jets t¯t events, includingthose with no correct jet-parton combination. Considering these kind of events, theefficiency of selecting the correct combination with the use of the Likelihood method,when no cut is applied on the Likelihood response, reduces to about 17% which isobtained as 34% × 51% ∼ 17%.5.1.6 Reconstructing W Boson and Top Quark MassesNow that the reconstructed jets are associated to the hadronic quarks which areproduced in the decay products of the hadronic top quark of an e+jets t¯t event,t → Wb → q¯qb, one can reconstruct the W boson four-vector by summing the fourvectorsof the two light jets. The four-vector of the top quark can be subsequentlyreconstructed by adding the four-vector of the hadronic b jet to the reconstructedhadronic W boson. Furthermore, one can measure the invariant masses of the W bosonand top quark from their reconstructed four-vectors. In Figure 5.9, the masses ofthe W boson and the top quark are shown for those events where the MVA successfullyreturned the true jet-parton combination. It is observed that the reconstructedW boson mass is peaked around 89 GeV while the parameter which has been usedfor the W boson mass when simulating the events was chosen to be 80.4 GeV. Alsothe mean value of the distribution of the top quark mass is shifted from the valueused in the simulation procedure, being 172.5 GeV, to a reconstructed value which isfound to be about 181 GeV. One should note that the events which are used to fill themass distributions with, are those events where the MVA successfully chose the truejet combination. If all selected events were taken into account, tails would appear onboth histograms.This observation leads to the fact that, the jet energies which are used to build the Wboson and top quark four-vectors are over-estimated which yields the obtained higherinvariant mass values compared to the input masses. Therefore this motivates to build