臺灣博碩士論文加值系統

我們在此呈現一次針對激發態輕子於2e2μ末態之搜尋結果。此分析為CMS實驗在尋找激發態輕子於四輕子末態實驗之部分。此次實驗利用LHC於√s = 8 TeV撞擊之數據。數據的蒐集由CMS偵測器完成，使用數據之累積照度達到19.7 fb-1。激發態輕子(l^*)預期將透過接觸相互作用與一標準模型之輕子伴隨生成。而後，激發態輕子將以l^*→lZ→lll (l=e,μ)的機制衰變，造成四輕子之末態。這是LHC第一個針對激發態輕子於四輕子末態之搜尋。實驗結果並未超出標準模型背景的預期。激發態輕子的預期生成截面積受到T_3耦合因子f以及Y耦合因子f'的選擇影響。對於2e2μ之末態，當f/f^'=1 (f/f^'=-1)及在Λ=M_(l^*)時，激發態電子與渺子之質量上限皆設在1.60(1.85) TeV於95%的信心水準。結合4e與4μ之實驗結果，激發態電子與渺子之質量上限可分別設在1.70(1.95) TeV以及1.75(2.00) TeV。

A search of excited lepton in 2e2μ final state is performed using a data sample of pp collisions at a center-of-mass energy √s = 8 TeV collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 19.7 fb-1. This is a part of the excited lepton search in 4-lepton final state in CMS. Excited leptons (l^*) are assumed to be produced via contact interactions in conjunction with a standard model lepton and to decay via l^*→lZ→lll (l=e,μ), yielding a final state with four
leptons. We present the first study of the search for excited lepton in 4-lepton final state at LHC. No signicant excess is observed in the data above the expected standard model background. The expect cross section of l^*can be signicantly affected by different choice of coupling constant f and f' to T_3 and Y. For f/f'=1(f/f'=-1),
upper bounds at 95% confidence level are set to 1.60(1.85) TeV for both e^* and μ^* in 2e2μfinal state with Λ=M_(l^*). Combining these results with 4e and 4μ channels, the limits can be set to 1.70(1.95) TeV and 1.75(2.00) TeV for e^* and μ^*, respectively.

1 Introduction and theory overview 1
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Theory overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Model of excited leptons . . . . . . . . . . . . . . . . . . . . . 2
1.2.2 Decay of excited leptons . . . . . . . . . . . . . . . . . . . . . 5
1.2.3 Production of excited leptons . . . . . . . . . . . . . . . . . . 8
2 CMS detector and LHC 11
2.1 Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 CMS detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Coordinate in CMS . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.2 Superconduction solenoid . . . . . . . . . . . . . . . . . . . . 14
2.2.3 Tracking system . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.4 Electromagnetic calorimeter . . . . . . . . . . . . . . . . . . . 17
2.2.5 Hadron calorimeter . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.6 Muon chamber . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3 Trigger system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3 Analysis Procedures 23
3.1 Data and Monte Carlo samples . . . . . . . . . . . . . . . . . . . . . 23
3.1.1 Data samples . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1.2 Monte Carlo samples . . . . . . . . . . . . . . . . . . . . . . . 23
3.2 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.3 Object reconstruction and identification . . . . . . . . . . . . . . . . 28
3.3.1 Electron reconstruction . . . . . . . . . . . . . . . . . . . . . . 28
3.3.2 Electron identification . . . . . . . . . . . . . . . . . . . . . . 30
3.3.3 Muon reconstruction . . . . . . . . . . . . . . . . . . . . . . . 33
3.3.4 Muon identification . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3.5 Pileup Reweighting . . . . . . . . . . . . . . . . . . . . . . . . 38
3.4 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.1 Analysis strategy . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.2 Background estimation . . . . . . . . . . . . . . . . . . . . . . 44
3.5 Systematic uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . 53
4 Results and conclusion 56
4.1 Cross section limit computation . . . . . . . . . . . . . . . . . . . . . 56
4.2 Optimization of the final selection . . . . . . . . . . . . . . . . . . . . 57
4.3 Limit computation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.3.1 Limits of excited muons for f = f' = 1 . . . . . . . . . . . 60
4.3.2 Limits of excited electrons e for f = f' = 1 . . . . . . . . . . 61
4.3.3 Limits for excited leptons for f = f' = -1 . . . . . . . . 63
4.3.4 Limits of excited leptons e for f = f' = -1 . . . . . . . . . 63
4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

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