Probing Quartic Gauge Couplings through Triple Massive Vector Boson Production at the ATLAS Experiment

Abstract

This thesis presents the observation of the massive tri-boson production process $VVZ$ ($V=W^\pm,Z$) and interprets this measurement as a probe of anomalous quartic gauge couplings within an effective field theory framework. The analysis is performed on the ATLAS full Run 2 proton-proton collision dataset at centre-of-mass energy 13 TeV, corresponding to an integrated luminosity of 140 $\textrm{fb}^{-1}$. Combining three final state channels, achieves an observation of $VVZ$ production with a statistical significance of 6.4$\sigma$, and measures the signal strength of the process to be \begin{equation} \mu_{VVZ} = 1.43 \pm 0.20~(\text{stat.})^{+0.21}_{-0.19} (\text{syst.}) \end{equation} corresponding to a measured inclusive cross-section of $660^{+93}_{-90}(\text{stat.})^{+88}_{-81}(\text{syst.})$ fb. Additionally, signal strengths and inclusive cross-sections are measured for the specific $WWZ$ and $WZZ$ tri-boson production modes. Evidence of the $WWZ$ tri-boson production mode is observed with a significance of 4.4$\sigma$. The \tl and \fl channels of the analysis are used to set 95\% confidence level limits on the Wilson coefficients of dimension-8 effective field theory operators which moderate the contributions of heavy new physics to multi-boson processes through anomalous quartic gauge couplings. These limits are set while preserving unitarity and constitute the first effective field theory limits set using massive tri-boson production by a LHC collaboration.

Date of Award	26 Aug 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	T. R. Wyatt (Co Supervisor) & Alexander Oh (Main Supervisor)