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研究生:高凱逸
研究生(外文):Kai-Yi Kao
論文名稱:在7和8TeV質心能量質子對撞中藉底夸克加Z波色子事件尋找成對產生的類向量夸克
論文名稱(外文):Search for the Pair-production of a Vector-like Quark in b + Z Events from pp Collisions at √s = 7 and 8 TeV
指導教授:侯維恕
指導教授(外文):Wei-Shu Hou
口試委員:李湘楠張寶棣何小剛熊怡陳凱風王名儒
口試日期:2013-06-18
學位類別:博士
校院名稱:國立臺灣大學
系所名稱:物理研究所
學門:自然科學學門
學類:物理學類
論文種類:學術論文
論文出版年:2013
畢業學年度:101
語文別:英文
論文頁數:94
中文關鍵詞:大強子對撞機緊湊渺子線圈類向量夸克
外文關鍵詞:LHCCMSVector-like quark
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這是個尋找成對產生類向量夸克B’ 的研究。所用的實驗數據是在大強子對撞機(LHC) 運轉時,分別在2011 年以7 TeV 和2012 年以8 TeV 質心能量的質子對撞並由緊湊渺子線圈(CMS) 偵測器取得,累積的數據量達4.9 fb−1 (2011),19.6 fb−1 (2012)。由底夸克標籤的粒子噴流和從電子對或渺子對重建的Z 波色子所組合的不變質量,其符合標準模型的預測。在2011 年的數據下,假設B’ 衰變為一個底夸克和一個Z 波色子的分支所占比率為100% 時,則有95% 的信心水準下預期質量下限為510 GeV/c2,而觀測所得下限為550 GeV/c2。2012 年更新的預期下限為680 GeV/c2 而觀測所得為700 GeV/c2。

A search is presented for the pair production of a vector-like quark, B’. Data corresponding to integrated luminosity of 4.9 fb−1 p from pp collisions at √s = 7 TeV in 2011 and 19.6 fb−1 at 8 TeV in 2012 collected with the CMS detector at the LHC were used. The invariant mass spectrum of b-tagged jets and Z boson candidates, with the latter identified through e+e− or μ+μ− decays, is examined for an excess of events and found to be consistent with the standard model expectations. The expected 95% confidence level lower limit on the mass of the B’ quark with the assumption a 100% branching ratio fraction into a bottom quark and a Z boson is 510 GeV/c2, while the observed lower limit is 550 GeV/c2 in 2011; For the updates in 2012, the expected limit is 680 GeV/c2 and observed lower limit is 700 GeV/c2.

1 Introduction 1
2 Detector 3
3 Reconstruction and Selection Criteria 5
3.1 Reconstructed B’ Candidates for 7 TeV . . . . . . . . . . . . 5
3.2 Reconstructed B’ Candidates for 8 TeV . . . . . . . . . . . . 6
4 Analysis for 7 TeV data 11
4.1 Event yield for 7 TeV . . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Systematic Uncertainty for 7 TeV . . . . . . . . . . . . . . . . 13
4.3 Mass Limits for 7 TeV . . . . . . . . . . . . . . . . . . . . . . 15
4.4 Summary for 7 TeV . . . . . . . . . . . . . . . . . . . . . . . . 16
5 Analysis for 8 TeV data 17
5.1 Background Estimation for 8 TeV . . . . . . . . . . . . . . . . 17
5.1.1 Method of background estimation . . . . . . . . . . . 18
5.1.2 MC Closure test . . . . . . . . . . . . . . . . . . . . . 20
5.1.3 Extrapolating test for data . . . . . . . . . . . . . . . 20
5.1.4 Estimated background . . . . . . . . . . . . . . . . . . 23
5.2 Systematic Uncertainty for 8 TeV . . . . . . . . . . . . . . . . 24
5.3 Mass Limits for 8 TeV . . . . . . . . . . . . . . . . . . . . . . 27
5.4 Summary for 8 TeV . . . . . . . . . . . . . . . . . . . . . . . . 29
6 Discussion for the analysis in 2011 33
6.1 Detailed Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.2 The evaluation of the event selection criteria . . . . . . . . . . 37
6.3 The interesting issues . . . . . . . . . . . . . . . . . . . . . . 39
7 Discussion for the analysis in 2012 41
7.1 Detailed Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
7.2 The evaluation of the event selection criteria . . . . . . . . . . 45
7.3 Event selection efficiency vs. branching ratio of B’ → bZ . . . 47
7.4 Details for using extraplating method in 7 TeV data . . . . . 53
7.5 Monte Carlo modeling issue . . . . . . . . . . . . . . . . . . . 62
7.6 The interesting issues . . . . . . . . . . . . . . . . . . . . . . 65
8 Conclusion 75
9 Service Work 77
9.1 Low voltage system . . . . . . . . . . . . . . . . . . . . . . . . 77
9.2 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
9.2.1 Resolution of Preshower . . . . . . . . . . . . . . . . . 79
9.2.2 Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . 79
9.2.3 Alignment for straight line prediction . . . . . . . . . 80
9.2.4 Alignment in 2010 . . . . . . . . . . . . . . . . . . . . 80
9.2.5 Alignment in 2011 and 2012 . . . . . . . . . . . . . . . 81

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