於緊湊緲子線圈質心能量十三兆電子伏特尋找激發態頂夸克成對生成事件

Search for excited top quark pair production in lepton plus jets final state with √s = 13 TeV at CMS

陳以牧(Yi-Mu Chen)

指導教授 ： 陳凱風

國立臺灣大學/理學院/物理學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201700871

摘要

本研究利用大強子對撞機上的緊湊緲子線圈偵測器所收集到質心能量 13 兆電子伏特的質子對撞事件中,尋找自旋 3/2 激發態頂夸克成對生成事件。在藍氏–桑氏模型中,激發態頂夸克將衰變成頂夸克和膠子。
本分析關注之事件為頂夸克對衰變成兩個底夸克與兩個 W 玻色子,其中一個 W 玻色子繼續衰變成的兩個夸克,另一個 W 玻色子衰變成帶電輕子 (電子或緲子) 及微中子。被納入分析之對撞事件,其最終態必須有一個孤立輕子、橫量動量不對稱、以及至少六個噴流;其中兩個噴流的性質必須與底夸克經強作用分裂所形成的噴流性質相容。
本分析嘗試將事件中的運動變量重建為激發態頂夸克物件,以重建質量作為信號、背景事件的判別變量,並藉最大似然回歸分析分離觀察資料之質譜中信號與背景事件數量。
緊湊緲子線圈偵測器在 2016 年蒐集之累積光度 35.9 逆飛靶恩的實驗數據,與標準模型預測並無顯著性差異。推斷在 95% 的信心水準下質量小於 1.2 兆電子伏特等效質量的自旋 3/2 激發態頂夸克不存在。

關鍵字

激發態頂夸克 ； 自旋 3/2 夸克 ； 輕子加噴流 ； 超弦共振

並列摘要

This research searches for the pair production of spin-3/2 excited top quarks (t* t-*) in an extension of the Randall-Sundrum model with each excited t* decaying to a top quark and a gluon, using the data collected in 2016 with the CMS detector from pp collisions with a centre-of-mass energy of 13 TeV, and an integrated luminosity of 35.9 fb−1 .
The search focuses on the semileptonic decay of the top quark pairs, with one top quarks decaying to a b quark plus two additional quarks from a hadronically decaying W boson, and the other top quark decaying to a b quark plus a lepton (muon or electron) and a neutrino from a leptonically decaying W boson. The observed final state of events that are selected for analysis requires an isolated muon or electron, an imbalance in transverse momentum, and at least six jets, two of which must be compatible with originating from the fragmentation of a b quark.
The strategy of the analysis focuses on a kinematic reconstruction of t* objects,
using the reconstruction mass as the discriminating variable for distinguishing between signal and background events. Extraction of potential signal events in data is obtained from an unbinned maximum likelihood fit of a signal-plus-background model to the observed mass spectrum. The data shows no significant excess over standard model predictions, and a lower limit of 1.2 TeV/c2 at 95% confidence level on the mass of the spin-3/2 t* quark model is set.

並列關鍵字

excited top quark ； spin-3/2 quark ； lepton plus jet ； string resonance

參考文獻

37 S. van der Meer, “Calibration of the effective beam height in the ISR”, CERN Technical Report CERN-ISR-PO-68-31, (1968).

1 CMS, “Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC”, Phys. Lett. B716, 30–61 (2012).

2 ATLAS, “Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC”, Phys. Lett. B716, 1–29 (2012).

3 ATLAS and C. Collaborations, “Combined Measurement of the Higgs Boson Mass in pp Collisions at √s = 7 and 8 TeV with the ATLAS and CMS Experiments”, Phys. Lett. 114, 191803 (2015).

4 S. L. Glashow, J. Iliopoulos, and L. Maiani, “Weak interactions with lepton-hadron symmetry”, Phys. Rev. D 2, 1285–1292 (1970).