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

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52 CHAPTER 4: Reconstructing and Selecting Physics Eventcalled generally background and should be removed in order not to enter the analysisand spoil the physics results.The description of the configuration of the full signal event in this study is the subjectof the next section.4.2.1 The Signal Signature in the DetectorTop quark decays almost exclusively to a W boson and a b quark which subsequentlyhadronizes and produces a jet in the detectort(¯t) → W + (W − ) + b(¯b).Hence the decay products of a pair of top quarks only depend on the various W bosondecay channels as listed in Table 4.1 [94, 95].W decay modeBranching RatioW → lν e ; l = e, µ, τ. ∼ 33%W → q¯q; q¯q = u ¯d, c¯s. ∼ 66%Table 4.1: Various W decay channels and their branching ratio.If both of the Ws coming out of the top quarks decay into leptons and their correspondingneutrinos, it is called a full leptonic decay mode of t¯t system. If only one ofthe Ws decays leptonically and the other goes to hadrons, this makes a semi-leptonicdecay mode of a top quark pair. Otherwise, a full hadronic t¯t decay mode correspondsto the situation when both of the W bosons decay hadronically. The branching ratiosof the different decay modes of t¯t event are summarized in Table 4.2.t¯t decay modeBranching RatioFull leptonicSemi leptonicFull Hadronict¯t → W + bW −¯b → l + l − b¯b + E missT ∼ 11%t¯t → W + bW −¯b → lq¯qb¯b + EmissT ∼ 45%t¯t → W + bW −¯b → q¯qq¯qb¯b ∼ 44%Table 4.2: Various t¯t decay channels and their branching ratio.
CHAPTER 4: Reconstructing and Selecting Physics Event 53In this analysis, the semi-leptonic decay channel of a t¯t system with the lepton beingan electron is considered as signal and a typical signal event is depicted in Figure 4.5.Figure 4.5: Observed topology of a generic e+jets decay of a t¯t system.The final state of a e+jets t¯t decay, contains an isolated electron, at least four jetsas a result of the hadronization of the four quarks of which two of them originate fromheavy partons and missing transvers energy due to the presence of a neutrino amongthe decay products.4.3 Simulated Signal and Background Monte CarloSamplesThe simulation of all signal and background events was performed using the MadGraphgenerator except for QCD samples which were produced using PYTHIA. The MLMmatching scheme was used to perform the matching between the partons generatedby MadGraph and the parton showering from PYTHIA. In addition to the production oftop quark pairs via the strong interaction, single top quarks which are produced viaelectro-weak interactions have similar signatures as the signal events and are consideredas backgrounds. Also W and Z bosons in association with extra jets, where the vectorboson decays leptonically, can mimic the final state of semi-leptonic t¯t events. Due tothe large cross section of the W+jets process as well as the inclusive production of aprompt electron in the final state, these events constitute the main backgrounds to thesignal.The other main background are QCD multijet events for which due to the huge crosssection, it is hard to produce the enough amount of statistics which can cover the wholephase space. Also because of the fact that normally no prompt electron is producedin a generic QCD event, QCD events are suppressed and one would encounter a lackof QCD statistics in the analysis. Therefore in order to obtain reliable results, thereare some filters applied at generator level namely BCtoE and EMEnriched resulting inevents which are suitable for the analyses containing electrons. The first filter tries tosave those events containing electrons with at least 10 GeV of transverse energy thatare produced in the tracker acceptance. These electrons are asked to originate from ab or c quark so that events which are able to pass this filter are enriched with electrons
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Vrije Universiteit BrusselFaculteit
Page 5 and 6: IntroductionThe Standard Model of p
Page 7 and 8: Chapter 1The Standard Model of Part
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Page 71 and 72: Chapter 5Estimating of the Jet Ener
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CHAPTER 5: Estimating of the Jet En
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Chapter 6ConclusionThe Standard Mod
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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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