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Title Page 2

Abstract 5

Contents 6

1. Introduction 20

2. The CMS experiment 36

2.1. The Large Hadron Collider 36

2.2. The CMS detector 39

2.2.1. Coordinate System 41

2.2.2. Silicon Pixel and Strip detector 42

2.2.3. Electromagnetic calorimeter 43

2.2.4. Hadronic calorimeter 45

2.2.5. Solenoid magnet 47

2.2.6. Muon detector 47

2.3. Reconstruction and Identification 49

2.3.1. Electrons 49

2.3.2. Muons 49

2.3.3. Missing transverse energy 51

3. Data and Monte Carlo samples 53

3.1. Data 53

3.1.1. Triggers 53

3.1.2. Datasets 54

3.2. Monte Carlo samples 55

3.2.1. Signal Monte Carlo samples 55

3.2.2. Background Monte Carlo samples 56

3.3. Control datasets and Monte Carlo samples 57

4. Boosted signatures 59

4.1. Overlapping isolation cones 59

4.2. Overlapping ECAL clusters 60

4.3. Merged ECAL clusters 63

4.4. Cleaned inner tracks 65

5. Modified high-energy electron identification 68

5.1. Secondary track selection 68

5.2. Modified isolation criteria 69

5.3. Modified track-cluster matching 75

5.4. Modified high-energy electron ID efficiency 76

6. Merged electron identification 85

6.1. Training input variables 85

6.2. Boosted Decision Tree training 88

6.3. Merged electron efficiency 90

6.3.1. Efficiency of the model with secondary tracks 91

6.3.2. Efficiency of the model without secondary tracks 93

6.4. Merged electron energy scale and resolution 94

7. Event selections 99

7.1. Baseline selections 99

7.2. Resolved channels 100

7.3. Merged channels 101

8. Background estimations 103

8.1. Resolved channels 104

8.1.1. Fake factors 104

8.1.2. Control regions 106

8.2. Merged channels 107

8.2.1. Merged electron fake factors 107

8.2.2. Merged electron control regions 113

8.2.3. Cleaned muon transfer factor 120

8.2.4. Cleaned muon sideband region 120

9. Signal modeling 124

10. Systematic uncertainties 128

11. Results 131

12. Conclusions 136

Bibliography 139

Appendices 150

Appendix A. Electron trigger efficiency 150

Appendix B. Boosted muon isolation efficiency 159

초록 164

List of Tables 18

Table 2.1. LHC beam parameters during Run 2 pp collisions 39

Table 2.2. Summary of the Phase 1 pixel detector parameters 43

Table 2.3. High-pT muon and tracker high-pT muon ID criteria[이미지참조] 50

Table 3.1. Triggers for collecting eeee final state events 53

Table 3.2. Triggers for collecting eeμμ and μμμμ final state events 53

Table 3.3. The primary datasets for collecting events from double electron... 54

Table 3.4. The primary datasets for collecting events from single muon... 55

Table 3.5. The list of signal samples. Samples with MY〈100GeV are...[이미지참조] 56

Table 3.6. The list of background samples and their cross sections 57

Table 3.7. The list of (upper) datasets and (lower) MC samples for Z → ee... 57

Table 3.8. The list of (upper) datasets and (lower) MC samples for J/ψ → ee... 58

Table 3.9. The list of (upper) datasets and (lower) MC samples for Z → μμγ... 58

Table 5.1. Secondary track selection criteria 69

Table 5.2. Modified HEEP ID criteria 80

Table 5.3. Selection criteria for tag & probe electrons and the trigger... 84

Table 6.1. List of variables used to train the model with secondary tracks 86

Table 6.2. List of variables used to train the model without secondary tracks 88

Table 6.3. Selection criteria for J/ψ → ee control region 92

Table 6.4. Selection criteria for single-leg conversion control region 95

Table 6.5. Additional selection criteria for the B → J/ψK → eeK control... 97

Table 7.1. The list of signal regions in the analysis. Each signal region consists... 99

Table 8.1. Summary table of signal and control regions 103

Table 8.2. The fitted parameters of the prompt merged electron fake factor... 109

Table 9.1. Signal region histogram bin edges for statistical tests 127

Table 10.1. The list of systematic uncertainty sources, their types, and... 129

List of Appendix Tables 19

Table A.1. The list of (upper) datasets and (lower) MC samples for measuring... 150

Table A.2. Tag & probe electron selection criteria for measuring the seeded... 151

Table B.1. The list of (upper) datasets and (lower) MC samples for measuring... 159

Table B.2. Tag & probe muon selection criteria for measuring the isolation... 160

List of Figures 9

Figure 1.1. A Feynman diagram of the leptophobic Z¹ decaying into four-... 31

Figure 1.2. Feynman diagrams of the 2HDM for (left) H → AA → 4ℓ and... 33

Figure 1.3. Feynman diagrams of dark photons ZD associated with a (left)...[이미지참조] 34

Figure 2.1. Schematic layout of the LHC and CERN accelerator complex as... 37

Figure 2.2. Schematic drawing of the CMS detector 40

Figure 2.3. Sketch of one quarter of the Phase-1 CMS tracking system in r-z... 44

Figure 2.4. Schematic drawing of the HCAL, including HB, HE, HF, and... 46

Figure 2.5. Schematic view of the CMS muon detector in the r-z plane.... 48

Figure 4.1. An event display of MH＝250GeV, MA＝10GeV signal MC...[이미지참조] 60

Figure 4.2. An event display of MH＝250GeV, MA＝1GeV signal MC...[이미지참조] 61

Figure 4.3. An event display of MH＝2000GeV, MA＝10GeV signal MC...[이미지참조] 62

Figure 4.4. An event display of MH＝250GeV, MA＝1GeV signal MC...[이미지참조] 63

Figure 4.5. An event display of MH＝250GeV, MA＝1GeV signal MC...[이미지참조] 64

Figure 4.6. An event display of MH＝250GeV, MA＝1GeV signal MC...[이미지참조] 65

Figure 4.7. An event display of MH＝2000GeV, MA＝1GeV signal MC...[이미지참조] 66

Figure 4.8. An event display of MH＝2000GeV, MA＝1GeV signal...[이미지참조] 67

Figure 5.1. Modified cone of the (left) tracker and (right) ECAL isolation. A... 70

Figure 5.2. The track isolation efficiencies in (upper left) EB and (upper... 71

Figure 5.3. The track isolation efficiencies in (upper left) EB and (upper... 72

Figure 5.4. The track isolation efficiencies in (upper left) EB and (upper... 73

Figure 5.5. The track isolation efficiencies in (upper left) EB and (upper... 74

Figure 5.6. Visual representations of the variable αtrack, Δuin5×5 and Δυin5×5,...[이미지참조] 76

Figure 5.7. The distribution of (left) Δηinseed and (right) Δuin5×5 variables for...[이미지참조] 77

Figure 5.8. The distribution of (left) Δηinseed and (right) Δuin5×5 variables for...[이미지참조] 78

Figure 5.9. The modified HEEP ID efficiencies in EB with (upper left)... 79

Figure 5.10. The modified HEEP ID data efficiency and SF as a function of... 81

Figure 5.11. The modified HEEP ID SF and its corresponding combined... 82

Figure 5.12. The modified HEEP ID SF and its corresponding combined... 83

Figure 6.1. Input variable distributions for the model with secondary tracks.... 87

Figure 6.2. Input variable distributions for the model without secondary... 89

Figure 6.3. Variable importance of the model (left) with and (right) without... 90

Figure 6.4. The simulated merged electron identification efficiencies as a... 91

Figure 6.5. The nominal dielectron mass distribution of J/ψ candidates in... 93

Figure 6.6. Efficiency and SF for the model with secondary tracks as a... 94

Figure 6.7. The nominal Z candidate mass distribution in data using μμγ... 96

Figure 6.8. Efficiency and SF as a function of (left) ET5×5 and (right) the...[이미지참조] 96

Figure 6.9. The invariant mass distributions of B → J/ψK → eeK... 97

Figure 8.1. Schematic diagrams for control and signal regions of (upper left)... 104

Figure 8.2. The number of expected events in the signal region as a function... 108

Figure 8.3. The merged electron fake factor for prompt electrons as a... 110

Figure 8.4. The merged electron fake factor for prompt electrons with ET5×5〈...[이미지참조] 111

Figure 8.5. The merged electron fake factor for prompt electrons with ET5×5〉...[이미지참조] 112

Figure 8.6. The merged electron fake factor for the nonprompt contribution... 114

Figure 8.7. The observed and expected number of events as a function of... 115

Figure 8.8. The corrected invariant mass distribution in the one-passing... 116

Figure 8.9. The merged electron's (left) ET5×5 and (right) η5×5 for the region...[이미지참조] 118

Figure 8.10. The expected number of events as a function of the invariant... 119

Figure 8.11. The expected and observed number of events in the 0.3〈... 122

Figure 8.12. The expected and observed number of events in the ｜Δφ｜〈0.3... 123

Figure 9.1. The acceptance times efficiency times branching fraction as a... 125

Figure 9.2. The acceptance times efficiency times branching fraction as a... 126

Figure 11.1. The observed and expected limit on the product of the cross... 132

Figure 11.2. The observed and expected limit on the product of the cross... 133

Figure 11.3. The 2-dimensional observed upper limit on the product of the... 134

Figure 11.4. The invariant mass distribution between two muons in the... 134

Figure 11.5. The signal + background fit for MX＝550GeV and MY ＝...[이미지참조] 135

List of Appendix Figures 16

Figure A.1. The efficiency and SF for the double electron trigger's seeded leg... 152

Figure A.2. The efficiency and SF for the double electron trigger's unseeded... 153

Figure A.3. (left) The trigger efficiency SF and (right) the corresponding... 154

Figure A.4. (left) The trigger efficiency SF and (right) the corresponding... 155

Figure A.5. (left) The trigger efficiency SF and (right) the corresponding... 156

Figure A.6. (left) The trigger efficiency SF and (right) the corresponding ... 157

Figure A.7. The nominal (red) signal and (blue) background shapes for... 158

Figure B.1. The isolation efficiency and SF estimated using boosted... 161

Figure B.2. The isolation efficiency and SF estimated using boosted muons... 162

초록보기

 LHC의 CMS 검출기를 이용하여 기록한 137.6 fb-¹ 의 양성자-양성자 충돌 데이터를 사용하여 두 개의 중간 상태 보존을 거쳐 최종적으로 네 개의 경입자로 붕괴하는 무거운 질량의 입자를 탐색하였다. X → YY → 4ℓ 과 X → ZY → 4ℓ 가설에 대한 생산 단면적의 상한을 계산하였다. 보존 Y의 질량이 작을 경우, 높은 로렌츠 부스트로 인해 인접한 두 개의 전자의 에너지 분포가 합쳐지거나, 검출기에 기록된 두 경입자의 궤도가 겹침으로써 일반적인 것과는 다른 신호가 관측될 수 있다. 따라서, 가벼운 중간 상태 보존을 거치는 가설의 경우 새로이 개발된 인접한 전자 및 뮤온 식별 기술을 사용하여 탐색하였다. 데이터 상에서의 인접한 전자-양전자 쌍에 대한 식별 효율은 J/ψ → ee 과 한 개의 광자가 두 개의 전자-양전자 쌍을 생성하는 Z → µµγ 이벤트들을 사용해 측정하였다. 인접한 뮤온의 궤도 겹침 현상이 발생했을 경우 missing transverse energy를 관측되지 않은 뮤온의 것으로 해석하였다. 유의미한 신호는 관측되지 않았다. 관찰된 최대 global (local) 통계적 유의성은 MX = 550 GeV 및 MY = 0.8 GeV 에서 1.84σ (2.74σ) 이다.

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