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A search for the standard model four top quark production in pp collisions at \sqrt{s}=13 TeV in CMS

A search for the standard model four top quark production in pp collisions at \sqrt{s}=13 TeV in CMS

초록/요약

We search for the standard model (SM) production of four top quarks (\mbox{$t\bar t$$t\bar t$}) using multi-lepton events ($l=e,\mu$) containing a same-sign pair or at least three leptons. Events are produced in proton-proton collisions with $\sqrt{s} = 13\; \mathrm{TeV}$ at the LHC, and the data sample, recorded using the CMS detecter in 2016, corresponds to an integrated luminosity of $35.9\; \mathrm {fb^{-1}}$. We utilize a boosted decision tree to separate \mbox{$t\bar t$$t\bar t$} events from the dominant SM backgrounds. The number of events observed after all selection requirements is consistent with the expectations from the background predictions, and we set an upper limit on the cross section for \mbox{$t\bar t$$t\bar t$} production in the SM of $23.8\; \mathrm{fb}$ at 95\% confidence level ($2.6 \times \mathrm{the\; prediction\; for\; the\; signal}$), with an expected limit of $19.3^{+11.5}_{-6.8}\; \mathrm{fb}$.

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목차

1 Introduction 1
2 Theory 5
2.1 The Standard Model of Particle Physics 5
2.1.1 Fermions and bosons 5
2.1.2 Quantum electrodynamics 7
2.1.3 Quantum chromodynamics 8
2.1.4 Weak interactions 9
2.1.5 The final Lagrangian 12
2.2 Top Quark Physics 13
2.2.1 Top quark pair production 14
2.2.2 Single top quark production 14
2.2.3 Four top quark production 15
3 The Apparatus 17
3.1 Large Hadron Collider 17
3.2 Compact Muon Solenoid detector 20
3.2.1 Solenoid magnet 21
3.2.2 Inner tracking system 22
3.2.3 Electromagnetic calorimeter 23
3.2.4 Hadronic calorimeter 25
3.2.5 Muon system 27
3.2.6 Data acquisition system 29
4 Event Reconstruction and Object Selection 31
4.1 Track reconstruction 32
4.2 Primary vertices 32
4.3 Particle flow 33
4.4 Lepton multi-isolation 34
4.5 Muons 35
4.6 Electrons 36
4.7 Jets 38
4.8 b-tagging 39
4.9 Missing transverse energy 40
5 Data and Simulation Samples 41
5.1 Collison data and triggers 41
5.2 Monte Carlo simulations 43
5.3 Residual corrections 48
6 Analysis Techniques 50
6.1 Imulti study 50
6.1.1 Imini and PFIsoBeta 51
6.1.2 pTrat.and pTrel. 53
6.1.3 Imulti performance study 55
6.2 Boosted Decision Tree 57
6.3 Trijet selection 58
6.4 Systematic uncertainties 65
6.5 Higgs Combined Tool 66
7 The Same-Sign Di-lepton (SSDL) Analysis 70
7.1 Baseline selection and event yields 70
7.2 BDT analysis 74
7.3 Systematic uncertainties 92
7.4 Limit setting 104
8 The TriOrMore Lepton Analysis 107
8.1 Baseline selection and event yields 107
8.2 BDT analysis 111
8.3 Systematic uncertainties 128
8.4 Limit setting 140
9 The Combined Analysis 142
10 Conclusion 146
A The Cut and Count Analysis 147
A.1 Baseline selection 147
A.2 Control and signal regions 148
A.3 Event yields 149
A.4 Systematic uncertainties 151
A.5 Limit setting 165
B Limit Setting Tests Using the Higgs Combined Tool 166
B.1 Tests for the BDT analysis 166
B.2 Tests for the CnC analysis 169

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