利用ATLAS探測器在質心能量13 TeV質子-質子對撞實驗中尋在衰變至eμ,eτ及μτ末態的高質量中性粒子

In the Standard Model (SM) of particle physics, there are three lepton flavors--electron, muon and tau flavors. All physical process including particle decays under the framework of the SM must respect the flavor symmetry: there is no flavor-violating process in the SM. In this analysis, three Beyond the Standard Model (BSM) physics models have been considered: a model with an additional U(1) gauge symmetry (Z') giving rise to a heavy Z' gauge boson, models with extra spatial dimensions giving rise to quantum black holes (QBHs) and an R-parity-violating supersymmetric model (RPV SUSY).In the Z' model, the SM gauge symmetry group has been enlarged by adding an extra U(1) gauge group. Breaking this additional symmetry gives rises to a massive vector boson--Z' boson. The Z' gauge boson in this analysis was assumed to have non-vanishing non-family-universal couplings such that lepton-flavor-violating decays were not forbidden. The Z' boson was assumed to have the same quark couplings as the SM Z boson, and Z'→eμ or eτ or μτ couplings were non-zero.To incorporate gravity into the SM, a quantum theory of gravity is essential. In this analysis, two Quantum Gravity models have been considered--the Arkani-Dimopoulos-Dvali (ADD) model with 6 extra extra spatial dimensions and the Randall-Sundrum (RS) model with 1 extra spatial dimension. The mass scale of the Quantum Gravity has been set to the Planck scale, 10^ GeV, that is far apart from the mass scale of the electroweak symmetry breaking, 100 GeV. Introducing extra spatial dimension(s) to lower the value of the Planck mass is a solution of this hierarchy problem. Differing from the thermal black holes that undergo Hawking radiation, the quantum black holes are bound states of gravitons and do not radiate thermally. In QBH models, there were two main assumptions: gravity coupled with equal strength to all SM particle degrees of freedom, and quantum black holes decayed predominantly to two-particle final states.In the RPV SUSY model, four gauge-invariant terms have been introduced into the superpotential. three of them violate the lepton number by 1, whereas the remaining one violates the baryon number by 1. In this analysis, two cases were separately considered: lepton number was violated while baryon number was conserved; lepton number was conserved while baryon number was violated. Consequently, proton decay was avoided even if the R-parity was violated. Also, it was assumed that only the τ sneutrino was responsible for the lepton-flavor-violating decays.In this work, data collected by the ATLAS detector in 2015-2018 with integrated luminosity 139 fb^{-1} at √s=13 TeV, and Monte-Carlo (MC) simulated samples were used. Several SM background processes are relevant for this search: Drell-Yan, ttbar, single top, multijets, W/Z+jets and diboson processes. The main reducible backgrounds are multijets and W+jets processes. In general, detector simulation is not accurate enough for estimating the reducible backgrounds so data-driven methods were employed: matrix method for the eμ channel, and charge-based method and a tau fake rate method with side band method for eτ and μτ channels. Due to low statistics, the top background (ttbar and single top were combined) was estimated in the high mass region using an extrapolation method.Since there was no significant deviation from the SM prediction, exclusion limits on the cross-section times branching ratio (σBR) were extracted for all three channels using modified frequentist approach. The 95% confidence level (CL) expected upper limits for the Z' model were computed.在粒子物理學標準模型中，有三種不同的輕子味----電子(electron)、渺子(muon)和濤子(tau)。在標準模型的框架下，所有物理過程包括粒子衰變都必須遵從輕子味量子數守恆，所以並不存在輕子味量子數守恆破缺的物理過程。在這分析中，三個超越標準模型的物理模型被列入考慮，包括帶有額外$U(1)$規範對稱性並會於該對稱性破缺時產生出Z'玻色子的模型、帶有額外空間維度令量子黑洞能夠順利於大型強子對撞器中產生的模型和R宇稱破缺超對稱模型。在Z'玻色子的模型中，透過引入額外的U(1)規範對稱群，標準模型的規範對稱群被擴張。而當額外的規範對稱性出現破缺時，會產生出高質量的玻色子----Z'玻色子。在這分析中，因為假定了Z'玻色子有非零的輕子味不普遍性耦合，所以Z'玻色子能衰變至輕子味量子數不守恆的末態。這分析同時假定了Z'玻色子的夸克耦合跟標準模型中的Z玻色子一樣。為了整合標準模型跟重力理論，人們需要有一套描述重力的量子理論。這分析考慮了兩套帶有額外空間維度的理論，包括帶有6個額外空間維度的ADD理論和第有1個額外維度的RS理論。額外的空間維度大幅減低了量子重力對能量量級的要求，由原來的普朗克能量量級(10^ GeV)降至大型強子對撞器可達到的能量量級。這有望解決級列問題。有別於一般會發放霍金輻射的黑洞，這量子重力理論會產生出量子黑洞，為重力子的束縛態。在這分析中，假定了重力跟所有標準模型粒子的耦合強度相等和量子黑洞的主要末態為雙粒子末態。在R宇稱破缺超對稱模型中，在超級勢中引入了4項帶有規範不變性的項，當中3項會令輕子數改變正一或負一，另外1項會令重子數改變正一或負一。在這分析中，每個衰變過程中只能夠有輕子數或重子數的改變。這保證了質子不會因為R宇稱破缺而導致其衰變。另外，這分析中假定了只有濤超中微子作為虛粒子參與衰變過程。這分析使用了ATLAS探測器於2015至2018年間所收集到的質子----質子對撞數據，積分亮度為139 fb^{-1}，質心能量為13 TeV。分析中同時使用了Monte-Carlo模擬數據。在這次的分析中，主要的背景物理過程為Drell-Yan、ttbar、單一t夸克、multijets、W/Z+jets和雙玻色子過程。一般而言，模擬數據對這些的背景過程並不準確，所以需要運用到一些由實驗數據主導的研究方法。另外，因為t夸克相關的背景過程數據量不足，所以分析中運用了外推法。最後，因為在這次分析中並沒有發現跟標準模型預測有不吻合的結果，所以亦為物理量σBR定下了排阻極限，並以Z'模型為示範例子。KWOK, King Wai.Ph.D. Chinese University of Hong Kong 2021.Includes bibliographical references (leaves )Abstracts in English and Chinese.Title from PDF title page (viewed on ...