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Title: Search for the standard model production of a single top quark in association with a Z⁰ boson using machine learning techniques
Author: Hoad, Corin J. K.
ISNI:       0000 0004 9353 9143
Awarding Body: Brunel University London
Current Institution: Brunel University
Date of Award: 2020
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This thesis presents a search for the production of a single top quark in association with a Z⁰ boson in the dileptonic decay channel as predicted by the Standard Model. The search uses 77.8 fb⁻¹ of data from √s = 13 TeV proton-proton collisions collected by the Compact Muon Solenoid experiment at the Large Hadron Collider during the 2016-2017 data-taking period. The search identified events containing a Z⁰ boson decay by requiring two opposite-sign same-flavour electrons or muons in the final state with invariant mass compatible with the nominal Z⁰ boson mass. Products of the top quark decay were identified using techniques developed to identify jets originating from bottom quarks and searching for a jet pair with invariant mass compatible with the W± boson mass. Machine learning techniques were used to further discriminate the signal process from background events. A study was carried out, comparing the performance of boosted decision trees with hyperparameters optimised using a Gaussian process and multi-layer perceptrons on this problem. The boosted decision trees were found to outperform the multi-layer perceptrons. A signal strength of r̂ = 6.52+2.30 -2.05 was observed, where r̂ = 1.0 corresponds to the Standard Model expectation. The corresponding observed (expected) significance is 3.12σ (0.48σ).
Supervisor: Cole, J. Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: high energy physics ; particle physics ; boosted decision tree ; multilayer perceptron ; Gaussian process