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

Abstract

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”

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”