Searches for BSM physics with four top quarks

质子-质子碰撞中同时产生四个顶夸克是粒子物理标准模型预测的最罕见的过程之一。顶夸克的高质量意味着它与希格斯玻色子有着密切的关系，许多超越标准模型的理论预测了这一过程的增强截面。因此，寻找四个顶夸克的产生是发现新物理学的理想探测器，通过区分信号和背景来寻找这样的事件是一个挑战。因此，博士项目的目的是找到证据，或进一步限制与四个顶夸克的产生相关的BSM物理的存在。该项目属于STFC粒子物理研究领域的福尔斯项目。质子-质子碰撞中同时产生四个顶夸克（除非进一步说明，否则为四个顶夸克）是粒子物理标准模型（SM）预测的最罕见的过程之一。顶夸克的高质量意味着它与希格斯玻色子有着密切的关系，希格斯玻色子可以改变这个通道发生的频率，因此使四个顶夸克的产生及其随后的行为成为新物理学的理想探测器。许多超越标准模型（BSM）的理论（例如，亲顶矢量共振）也预测了该过程的增强的横截面。该项目的目的是寻找与四个顶夸克的产生有关的BSM物理学存在的证据或进一步限制其极限。顶夸克通过衰变成终态粒子、B夸克和W玻色子来观察，后者可以进一步衰变成夸克-反夸克对或带电轻子和中微子对。每个夸克还产生一个独特的粒子锥，称为喷流。产生顶夸克的方法有很多种，最常见的是夸克-反夸克对，但偶尔也会单独发现。事实上，四个顶的产生速率预计比产生顶夸克-反夸克对的速率低70，000倍。因此，观察难以捉摸的四顶信号是一个挑战，因为很难将其与背景过程区分开来，并且由于其不同的签名。机器学习的使用在许多方面彻底改变了粒子物理学，并在事件分类的几个分析中取得了成功。因此，主要目标是探索和确定采用哪些尖端机器学习技术来将信号与背景分离。然后将其用于对真实的数据进行物理分析，考虑有利信号通道中的所有系统和统计不确定性。该项目将与CERN的紧凑型μ子螺线管（CMS）实验合作进行，福尔斯STFC粒子物理研究领域的职权范围。这与STFC通过高级别问题C回答其科学挑战的物理学战略是一致的：“物质的基本成分是什么？它们如何相互作用？".更具体地说，这项工作通过问题C1和C2：“什么是基本粒子和场？“和“什么是物理学的基本定律和对称性？".这项工作将是英国布里斯托大学和德国DESY之间合作的产物。

The simultaneous production of four top quarks in proton-proton collisions is among the rarest processes predicted by the standard model of particle physics. The high mass of the top quark means it has a close relationship to the Higgs boson, and many beyond the standard model theories predict an enhanced cross-section of this process. Searching for four top quark creation is therefore an ideal probe to discover new physics, with the challenge arising in finding such events by distinguishing between signal and background. Hence, the PhD project aims to find evidence for, or further constrain limits of, the existence of BSM physics in association with the production of four top quarks. Cutting-edge machine learning techniques are to be explored and employed in disentangling the complex signature.This project falls under the remit of the STFC's particle physics research area.The simultaneous production of four top quarks (four-tops unless further specified) in proton-proton collisions is among the rarest processes predicted by the standard model (SM) of particle physics. The high mass of the top quark means it has a close relationship to the Higgs boson, which can change the frequency of this channel taking place, and therefore makes the creation of four top quarks and their subsequent behaviour an ideal probe for new physics. Many beyond the standard model (BSM) theories (e.g., top-philic vector resonances) also predict an enhanced cross-section of this process. The aim of the project is to find evidence for, or further constrain limits of, the existence of BSM physics in association with the production of four top quarks.Top quarks are observed through their decays into final state particles, the b quark and W bosons, where the latter can further decay into quark-antiquark pairs or charged lepton and neutrino pairs. Each quark also produces a distinctive cone of particles called a jet. There are a variety of ways to produce top quarks, which are most commonly found as quark-antiquark pairs but can occasionally be found on their own. In fact, the rate of production of four-tops is predicted to be 70,000 times lower than that of producing top quark-antiquark pairs. Observing the elusive four-tops signal is therefore a challenge due to the difficulty of distinguishing it from background processes, and due to its varied signature. The use of machine learning has revolutionised particle physics in numerous aspects, and has found success in several analyses with regards to event classification. Hence, a primary objective is to explore and determine which cutting edge machine learning techniques to employ for disentangling signal from background. This is then to be used for the physics analysis on real data, considering all systematic and statistical uncertainties in favourable signal channels. This is all to be done computationally.The project is to be conducted in collaboration with the Compact Muon Solenoid (CMS) experiment at CERN and falls under the remit of the STFC's particle physics research area. This is in accordance with the STFC physics strategy in answering its science challenges through high-level question C: "What are the basic constituents of matter and how do they interact?". More specifically, this work addresses whether there are discrepancies in what is predicted in the SM and what is observed, through questions C1 and C2: "what are the fundamental particles and fields?" and "what are the fundamental laws and symmetries of physics?". This work will be the product of a partnership between the University of Bristol, UK and DESY, Germany.