Searches for BSM physics with four top quarks

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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.