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

CHAPTER 3: Object Reconstruction 35online triggering system.In this study, only anti-k T jets are processed for which the R parameter is set to be0.5.3.2.2 Jet Reconstruction PerformanceIn order to evaluate the performance of the jet reconstruction algorithms, the createdCaloJets are compared to the particle level jets, the so-called GenJets, which are independentof the response of the CMS detector. GenJets are made by applying thesame jet reconstruction algorithm on the stable particles which are coming out of thehadronisation process. The GenJets are not observable jets, because they can only beaccessed from the simulated information. Equivalently, the actual detected jets areCaloJets which are GenJets right after they reach the detector. The jet matching efficiencywhich is defined as the fraction of the GenJets that match to the CaloJets withina ∆R cone of 0.5, is shown in Figure 3.3 for various jet clustering algorithms [69].Figure 3.3: Efficiency of matching a reconstructed CaloJet to a GenJet as a functionof p GenJetT for various jet clustering algorithms [69].According to Figure 3.3, the probability of finding calorimeter jets that match togenerated jets with, for example, p T = 30 GeV is larger than 95%. The drop in theefficiency at low p T is due to the presence of the magnetic field. Since low-p T chargedhadrons have a smaller bending radius, therefore they escape from the cone which iscast arount the jet and results in a wrong determination of the direction of the jet.Another parameter that describes how well a jet clustering algorithm works, is the