於 CMS 實驗質心能量 13TeV 質子對撞資料中尋找頂夸克衰變到 Z 玻色子與輕夸克之變味中性流事件

Search for Flavor Changing Neutral Currents Decays in t→qZ at centre-of-mass energy of 13 TeV in CMS

林俊廷(Chun-Ting Lin)

指導教授 ： 陳凱風

國立臺灣大學/理學院/天文物理研究所/碩士(2020年)

https://doi.org/10.6342/NTU202000610

摘要

本研究旨在尋找頂夸克經由變味中性流衰變之事件，而此衰變事件在標準
模型中是被GIM 機制所高度抑制的。其中，頂夸克經此機制衰變至一個上夸克
(或魅夸克)與一個Z 玻色子的衰變比率被理論預測約為10−14數量級。然而，根
據許多標準模型的延伸理論預測，此衰變比率是可以提升至10−4數量級的。
本研究中將會著重分析頂夸克與反頂夸克對事件，其中一個頂夸克會衰變
至一個底夸克與一個W 玻色子，而另一頂夸克將會衰變至一個上夸克(或魅夸
克)與一個Z 玻色子，而W 和Z 玻色子都將會衰變成輕子對。本研究所分析的數
據為緊湊緲子線圈偵測器在2016 與2017 年所蒐集的質心能量為13 兆電子伏特
之質子質子對撞數據，其合計總通量為77.4fb−1。經由本研究所得知，無顯著
超量的期望訊號事件數，而經由計算可得頂夸克衰變至一個上夸克(或魅夸克)
與一個Z 玻色子的預期衰變比率在2016 年之數據中不超過0.027%(0.032%)，
而在2017 之數據中不超過0.021%(0.025%)。
關鍵字詞: “變味中性流”， “頂夸克”， “緊湊緲子線圈偵測器”

關鍵字

變味中性流 ； 頂夸克 ； 緊湊緲子線圈偵測器

並列摘要

This analysis searches for the top quark decay through the flavor changing neutral
current (FCNC) process which is highly suppressed by the Glashow-Iliopoulos-
Maiani (GIM) mechanism in Standard Model (SM) and the branching fraction for a
top quark decaying into a charm or up quark and a Z boson is predicted to be of the
order 10−14. However, several extensions of the SM theories predict the branching
fraction of FCNC can be enhanced up to the order of 10−4 . The data collisions is collected
with the CMS detector in 2016 and 2017 from pp collision at the centre-of-mass
energy of 13 TeV and an integrated luminosity of 77.4 fb−1. This analysis focuses
on the top quark-antiquark events in which one of the top quark decays to a bottom
quark and a W boson and the other top quark decays to a light quark(u, c) and a Z
boson. Both the W and Z boson will decay into lepton pairs. No significant excess
is observed in the data. The expected upper limits are set on the branching fraction
of the top decays: Br(t −! uZ) < 0.027% and Br(t −! cZ) < 0.032% for 2016 and
Br(t −! uZ) < 0.021% and Br(t −! cZ) < 0.026% for 2017 at the 95% confidence
level.
Keyword : ”FCNC”, ”top quark”, ”CMS”

並列關鍵字

FCNC ； top quark ； CMS

參考文獻

[1] “The LHC Schematic”. https://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/lhc_in_pictures.htm. Accessed: 2020-01-04.

[2] S. L. Glashow, J. Iliopoulos, and L. Maiani, “Weak Interactions with Lepton-Hadron Symmetry”, Phys. Rev. D2 (1970) 1285–1292, doi:10.1103/PhysRevD.2.1285.

[3] Particle Data Group Collaboration, “Review of Particle Physics”, Phys. Rev. D 98 (Aug, 2018) 030001, doi:10.1103/PhysRevD.98.030001.

[4] J. A. Aguilar-Saavedra, “Top flavor-changing neutral interactions: Theoretical expectations and experimental detection”, Acta Phys. Polon. B35 (2004) 2695–2710, arXiv:hep-ph/0409342.

[5] CMS Collaboration, “Search for flavour changing neutral currents in top quark production and decays with three-lepton final state using the data collected at sqrt(s) = 13 TeV”,.