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

64 CHAPTER 4: Reconstructing and Selecting Physics EventThe multiplicity of the selected jets per event which already passed the Muon Vetocut, is shown in Figure 4.16. Those events with more than or equal to four selectedjets, are accepted for further analysis.#Events410310tt+jets semi-ele (1019.3 entries)tt+jets other (221.7 entries)t+jets, (all channels) (146.0 entries)Z+jets (1039.5 entries)W+jets (5308.2 entries)Multi-jets (12129.1 entries)210101-1100 1 2 3 4 5 6 7 8 9Nr. of Selected JetsFigure 4.16: Multiplicity of the selected jets per event which pass the Muon Veto cut.At Least One B-Tagged JetAs there are two b jets produced in the final state of the signal topology, it can beused as a discriminator between signal and backgrounds. In order to identify b quarkjets, various b-tagging algorithms have been defined within the CMS collaboration. Asimple one is called the “Track Counting High Efficiency” algorithm [99] which exploitsthe long lifetime of the b mesons produced in the hadronization process of the b quarks.Because of this property, the tracks which are associated to a b quark jet, are usuallydisplaced with respect to the primary vertex.The algorithm to tag b jets starts by selecting good quality tracks and associatingthem to a jet by a matching procedure. In a first step, the track impact parameter IPis computed by the algorithm, which is defined as the distance between the primaryvertex and the linearised track in the point of minimum distance between the trackand the jet axis. This is followed by the calculation of the track impact parametersignificance IP S , which is defined as the impact parameter divided by the resolution ofthe impact parameter σ IP , hence IP S = IPσ IP. In the next step, the tracks associatedto a jet are ordered in decreasing IP S . Finally, the “Track Counting High Efficiency”algorithm returns the IP S assigned to the second track in the list of ordered tracks asthe b-tagging discriminant. The distribution of the b-tag discriminator value returnedby this algorithms, is shown in Figure 4.17. The events with at least four selected jetsare taken into account.Due to the fact that the efficiency of b-tagging is not high, and in order not to loosetoo many signal events, the event is asked to have at least one b-tagged jet amongthe four selected jets. A jet is defined as a b jet if the tagging value returned by the“Track Counting High Efficiency” algorithm exceeds 4. The multiplicity of b-taggedjets among the four leading jets for those events that have at least four selected jets is