Searching for anomalous gauge boson interactions using precision measurements of $ZZjj$ production and inclusive $W^\pm\gamma$ production at the ATLAS experiment

Abstract

This thesis presents two electroweak physics analyses, which use $pp$ collision data collected by ATLAS at $\sqrt{s}=13$ TeV, with an integrated luminosity of 140 fb$^{-1}$. The first analysis is based on the measurement of differential cross-sections of $ZZ(\rightarrow 4\ell)jj$ production. This final state is sensitive to the purely electroweak vector boson scattering process, with $Z$ bosons being produced from a quartic gauge coupling vertex. The differential cross-sections are used to search for the anomalous triple and quartic gauge couplings that are introduced by dimension-6 and dimension-8 operators in the Standard Model effective field theory. The best constraints on the strength of operators are obtained through an observable constructed from the combined invariant mass of the dijet and four-lepton system. A dedicated clipping-scan study is performed on the dimension-eight operators to avoid unitarity violation. The second analysis is the measurement of the differential cross-section of inclusive $W^\pm(\rightarrow \ell\nu)\gamma$ production, which is sensitive to the anomalous triple gauge couplings introduced by dimension-6 operators in the Standard Model effective field theory. The \oWtil and \oHWBtil operators violate $CP$, which can be probed by $CP$-odd observables. The construction of an angular $CP$-odd observable and a novel neural-network-based observable is presented. The data-driven background estimates of a jet faking a photon and an electron faking a photon are discussed in detail. In addition to these analyses, optimisation of full scan tracking in the software-based trigger in ATLAS for Run 3 is presented, which improves the timing performance while preserving a sustainable tracking efficiency. According to the performance check on the early Run 3 data collected by ATLAS, the average execution time of the optimised full scan tracking per call is around 1 second for the average number of pile-up interactions of 50. The tracking efficiency is measured to be 99\% for tracks with a transverse momentum higher than 2 GeV.

Date of Award	1 Aug 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andrew Pilkington (Supervisor) & Darren Price (Supervisor)