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

CHAPTER 5: Estimating of the Jet Energy Scale Calibration Factor 79chosen jet-parton combination returned by the MVA method would still be taken ascorrect. Since there is no interest in the reconstruction of the leptonic part of t¯t systemfor this analysis, there is no force applied on the MVA method to associate a jetcorrectly to the leptonic b quark in the final state. The “Bad” jet combination then isthe combination for which the MVA cannot associate, correctly, at least one hadronicquark in the t → Wb → q¯qb decay topolology, to a reconstructed jet in the four leadingjets found in the event. Although in some cases the chosen jet-parton combinationby the MVA, is the true combination, in about half of the cases the Likelihood Ratiomethod is not able to return the true jet-parton combination.# events900800700Good Jet CombinationsBad Jet Combinations60050040030020010000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Likelihood ResponseFigure 5.6: The maximum among twelve possible values returned by the LikelihoodRatio method per event. Only those selected e+jets events for which the hadronic jetsare matched to the hadronic quarks in t → Wb → q¯qb, are taken into account.Considering those selected events with a correct jet-parton combination, the plotwhich contains the purity versus efficiency for the different cut values on the Likelihoodresponse, is shown in Figure 5.7. Efficiency is defined as the fraction of the selectede+jets events with a right jet combination that pass the cut on the Likelihood Ratioresponse. Purity is defined to be the number of the selected e+jets events with a rightjet combination that pass the cut on the Likelihood response relative to the multiplicityof all selected e+jets events, including either a right or wrong jet combination, thatpass the cut on the Likelihood Ratio response.According to the Monte Carlo truth, in only 34% of the selected e+jets t¯t events,the three reconstructed jets among the four leading jets are matched, truly, to thehadronic quarks in t → Wb → q¯qb. As it is understood from Figure 5.7, the LikelihoodRatio method is able to successfully find the true combination in 51% of those eventswith a true jet-parton matching, given that no cut is applied on the Likelihood Ratioresponse. Comparing to a random selection of the true combination that yields aprobability of about 8% of selecting the right jet combination, the great improvementwhich is obtained when using the MVA method is obvious.