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

CHAPTER 4: Reconstructing and Selecting Physics Event 61h/e ∆η in ∆φ in σ iηiηbarrel endcap barrel endcap barrel endcap barrel endcapcut value 0.15 0.07 0.007 0.01 0.8 0.7 0.01 0.03Table 4.5: Required cut values on the identification variables of the selected electron.#Events410310tt+jets semi-ele (1053.2 entries)tt+jets other (360.7 entries)t+jets, (all channels) (154.2 entries)Z+jets (2121.1 entries)W+jets (5370.5 entries)Multi-jets (12280.1 entries)210101-110-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5Nr. of Loose Other ElectronsFigure 4.12: Multiplicity of secondary loose electrons per event which has already hadexactly one selected electron.Muon VetoThe background events from µ+jets t¯t decays contain one muon in their final states.Also a muon can be reconstructed in the final state of a W+jets event where theW boson decays to a muon and corresponding neutrino. Hence vetoing muons in thefinal state of the event would reject most of the backgrounds containing a reconstructedmuon. The distribution of the muon muliplicity for those events that already passed thesecond electron veto cut, is shown in Figure 4.13. The reconstructed muons are askedto have a p T exceeding 20GeV and |η| less than 2.1. Also they are required to fulfilla relative isolation value, which is defined as the sum of the energy deposits in a coneof 0.3 in the tracker and the calorimeter divided by the muon transverse momentum,below 0.05. It can be seen that most of the t¯t events other than signal occupy thesecond bin of the distribution corresponding to those events with one reconstructedmuon in the final state. Hence these kind of backgrounds are suppressed by vetoingthe existence of a reconstructed muon in the event.Only those events appearing in the first bin of the plot shown in Figure 4.13,corresponding to the events with N selµ = 0, are processed in the rest of analysis.