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

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54 CHAPTER 4: Reconstructing and Selecting Physics Eventappearing in the decay products of hadrons containing heavy quarks. The second filteraims to gather those events with isolated electrons at generator level. It looks for singleelectrons with at least 20 GeV of transverse energy within the tracker acceptance thatare isolated in both the tracker and calorimeter sub-detectors. Both filters are madein such a way that the QCD events surviving each filter should be used together. Forsimplicity QCD events are produced in different bins of ˆp T , which is defined as thecenter of mass energy of the two colliding partons. The detailed information of theMonte Carlo simulated sets which are processed in this study, are given in Table 4.3.Process Name of Monte Carlo Sample σ[pb] #Eventst¯tW+jets (W → lν)Z+jets (Z → l + l − )tQCD/TTbarJets-madgraph 157.5 1483404/WJets-madgraph 31314 10068895/ZJets-madgraph 3048 1084921/SingleTop_tChannel-madgraph 20.93 528593/SingleTop_tWChannel-madgraph 10.6 459589/SingleTop_sChannel-madgraph 1.4 402055/QCD_BCtoE_Pt20to30 108330 2491921/QCD_BCtoE_Pt30to80 138762 2475597/QCD_BCtoE_Pt80to170 9422.4 1119546/QCD_EMEnriched_Pt20to30 1719150 33355445/QCD_EMEnriched_Pt30to80 3498700 39479587/QCD_EMEnriched_Pt80to170 134088 5494911Table 4.3: Simulated signal and background samples that are processed in this study.All samples have the ending “/Spring10-START3X V26 S09-v1/GEN-SIM-RECO”which has been dropped in the table. The mentioned cross sections correspond tothe Next-to-Leading Order (NLO) values except for QCD samples where the quotedcross sections have been calculated at Leading Order (LO) [96]. The numbers are takenfrom a common reference page of the top quark analysis group in CMS [97].
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
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Vrije Universiteit BrusselFaculteit
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IntroductionThe Standard Model of p
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CHAPTER 6: Conclusion 129The method
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Bibliography[1] M. E. Peskin and D.
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BIBLIOGRAPHY 133[40] The ALICE Coll
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BIBLIOGRAPHY 135[76] The CMS Collab
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SummaryJets appear in the final sta
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Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

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