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

62 CHAPTER 4: Reconstructing and Selecting Physics Event#Events410310tt+jets semi-ele (1022.1 entries)tt+jets other (310.1 entries)t+jets, (all channels) (149.1 entries)Z+jets (1045.4 entries)W+jets (5314.5 entries)Multi-jets (12144.0 entries)210101-110-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5Nr. of Loose MuonsFigure 4.13: Muon multiplicity per event which passed the Second Electron Veto cut.At Least Four JetsAt the pre-selection step, all events are required to have at least four reconstructed jetsin their final states. By tightening the thresholds on the jet kinematics and also by applyingsome identification criteria, the probability of selecting the four jets originatingfrom the quarks coming out of the hard scattering process becomes higher. The eventis required to have at least four selected jets with p T greater than 30GeV. Also jets,in order to be labeled as selected, are asked to pass identification cuts that are brieflydescribed below.• f EM which is called the electromagnetic energy fraction and is defined as thefraction of the jet energy measured in the ECAL. This parameter is useful indifferentiating between electrons and jets since jets with a high value of f EMprobably are electrons with energy deposits also used in the jet reconstructionalgorithm. The upper limit of this variable is set to be 0.9. Also in order tosuppress those jets which are actually pure electronic noise, a lower limit equalto 0.05 is put on this parameter.• n90Hits is defined as the minimal number of reconstructed hits containing 90%of the jet energy and is another observable that is used for the jet identification.The selected jets are required to have a n90Hits variable exceeding 4.• f HPD is defined as the fraction of energy in the hottest HPD readout. The selectedjets are asked to fulfill f HPD < 0.98.The distributions of the various identification variables of all reconstructed jets forthose events that already passed the Muon Veto cut, are shown in Figure 4.14.Looking at the distribution of the electromagnetic energy fraction, there is a clearpeak at ∼1 representing the electrons which are re-reconstructed as jets. Another plotthat shows the jet collection includes electrons and therefore needs to be cleaned, isthe distance between the selected electron and the closest jet among all reconstructed