研究多向量玻色子「火球」特徵訊號與參與 CMS 畫素偵測器第一期升級計畫

Abstract

本論文包含了兩部份:重夸克「火球」訊號的研究,以及 CERN/臺灣/芬蘭團隊在CMS 桶狀畫素偵測器 (BPIX) 第一期升級計劃中,生產畫素模組 (pixel module) 的紀錄。 第一部份,動機源自於單純的提問:為何宇宙主要是由物質所組成而非反物質?這個物質-反物質不對稱的問題,需要額外的超越粒子物理標準模型外的電荷宇稱不守衡 (CP violation) 才有可能被完整解釋。而標準模型外的重夸克,像是第四代夸克、或是類向量夸克 (vector-like quark) 等粒子,就很有機會能夠提供更多的電荷宇稱不守衡的現象。當這些新夸克的質量很大的時候,其相應的湯川耦合 (Yukawa coupling) 強度預期也會相當地大;如果這個耦合效應可以導致夸克-反夸克對以某種熱平衡的方式湮滅的話,那麼「火球」訊號便可能發生。「火球」訊號預測了湮滅後所產生的粒子主要是縱向的向量波色子(longitudinal vector boson),且數量會非常多。我們採用 Madgraph5 所產生的事件樣本進行「火球」訊號的研究,並藉由 Delphes 的模擬,考慮了簡化的 CMS 偵測器環境。如果在由 LHC 質心能量為13 TeV 的質子-質子對撞所產生、積分亮度 (integrated luminosity)高達100 fb -1 的數據中發現「火球」訊號的話,那麼對於重夸克的質量小於1 TeV/c 2 的「火球」訊號,其顯著性將可達五個標準差。反之,如果在相同的條件沒有發現「火球」訊號的話,在 95% 的信心水準下,質量小於 1.35 TeV/c 2 的重夸克是不存在的。 第二部份,CMS 的 BPIX 第一期升級計劃在2011年便開始進行。這是為了配合 LHC的升級計劃,因應更高的瞬時亮度 (~2×10 34 cm −2 s −1 ) 以及事件的堆積效應 (pile-up effect) 而作的準備。其中一項重要的工作,是由歐洲五個研究團隊所負責的畫素模組生產。 CERN/臺灣/芬蘭團隊屬於其中的一組研究團隊,負責生產 BPIX 第三層所需的畫素模組的總數一半。關於在 CERN 製程的進行,包含模組的組裝 (module assembly) 以及品質的測試(qualification tests) 等,都被紀錄在這份論文當中。其中作者參與了 X 光品質測試,相關的量測過程與分析圖表亦收錄其中。我們在2016年八月完成了這項任務,總計貢獻了207片畫素模組供第三層 BPIX 使用,另外有額外3片畫素模組作為第四層 BPIX 的備份元件。

The thesis includes the study of the fireball signature of heavy quarks and the record of the CERN / Taiwan / Finland module production in CMS barrel pixel detector (BPIX) phase 1 upgrade. For the first part, in order to solve the known problem of a matter-dominant universe, new CP violation source beyond the standard model of particle physics is needed. Adding new heavy quarks, such as sequential 4th generation quarks or vector-like quarks, to the model is a promising candidate solution. When the mass of the new quark is very heavy and hence their associated Yukawa coupling can be very large, a new fireball-like signature may appear if their strong couplings lead to annihilation in a thermal way. The emitted particles from the annihilation could be mainly longitudinal vector bosons, and the multiplicity is exceptionally high. We present the study of such a 'fireball' experimental signature with the fast simulated samples produced by the Madgraph5 generator. The detector effect is modelded by the Delphes package assuming a simplified CMS detector model. If the signal is discovered with a data set corresponding to an integrated luminosity of 100 $fb^{-1}$ collected at the LHC 13 TeV pp collisions, a statistical significance of $5sigma$ can be reached if the mass of the new heavy quark is below 1 TeV/$c^2$. If no excess above the background from other standard model processes is seen, a limit on the new heavy quark mass can be set to 1.35 TeV/$c^2$ at 95\% confidence level. For the second part, the CMS BPIX phase 1 upgrade has been being on going since 2011 with the provision for the progress of the LHC upgrade plans (high-luminosity LHC) and the expectation of even larger instant luminosity ($sim 2 imes 10^{34} cm^{-2}s^{-1}$) and pile-up effect in near future. One of the important tasks is barrel pixel module production which is in charge of five different consortia in Europe. CERN / Taiwan / Finland is the consortium responsible to 176 barrel pixel modules which are one-half the quota required for the Layer-3 BPIX. The module production progressing in CERN, including the module assembly procedure as well as the qualification tests, are recorded. As the writer participated in the x-ray qualification tests, both the procedure and analysis result in x-ray tests would be particularly detailed out. In the end of our module production in August 2016, there are 207 barrel pixel modules contributed to the Layer-3 BPIX, and additional 3 pixel modules to the backup modules for the Layer-4 BPIX.