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

CHAPTER 4: Reconstructing and Selecting Physics Event 554.4 Event SelectionTogether with signal events, at a proton-proton collider, there are many other kind ofevents produced of which some mimic the signal. For example, QCD multijet eventswhere a jet fakes an electron can produce a similar final state as e+jets t¯t events. Thesame reasoning can be made for a W+jets event when the W decays leptonically. AlsoZ → e + e − events of which one electron escapes detection would be another possiblebackground. Hence, in order to suppress all these background events, one can benifitfrom the kinematics of the signal and cut on the observables that differ between thesignal and its backgrounds. A detailed procedure of selecting e+jets t¯t events amongall other backgrounds is described in the next section.4.4.1 Selection CutsExpected Number of Events at the Center of Mass Energy of 7TeVAt 7TeV center of mass energy, the cross sections for signal and other relevant backgroundsare listed in Table 4.3. According toN = σLwhere L is the integrated luminosity and σ is the cross section for a specific kind ofprocess, N is the number of events expected after accumulating the data equivalent toL. The current study is based on L = 100pb −1 and the derived results are valid at thisregime of luminosity.Pre-SelectionIn order to reduce the size of the various simulated samples, a set of pre-selectioncuts is applied. All events are required to have at least one reconstructed electronand four reconstructed jets. Reconstructed electron and jets are asked to have a p Texceeding respectively 15 GeV and 20 GeV and to be within the tracker acceptance of|η| < 2.4. The p T and η distributions of all reconstructed electrons and jets for thoseevents surviving the pre-selection including all signal and backgrounds are shown inFigure 4.6.As it can be seen from Figure 4.6, the distribution of the pseudorapidity of theelectrons drop down in two parts which are representing the cracks between the ECALbarrel and the ECAL endcap. Also it can be understood that the jets in the semielectronict¯t events, are mainly reconstructed in the central part of the detector.Primary VertexIn order to make sure that the event which is under processing, comes out of a hardscattering and has not originated from pile-up interactions, some quality cuts are appliedon the reconstructed vertex of the event. Since one of the characteristics of pile-upcollision is a displacement primary vertex with respect to the nominal interaction vertex,one can ask each event to have a reconstructed primary vertex very close to the