Searches for new physics in vector boson scattering using the ATLAS experiment at the Large Hadron Collider

Abstract

This thesis presents the work of searches with the ATLAS detector for TeV-scale heavy Majorana neutrinos in same-sign $WW$ scattering using the $140~\textrm{fb}^{-1}$ of proton-proton collision data at $\sqrt{s} = 13~\textrm{TeV}$ delivered by the Large Hadron Collider. Three analyses were performed targetting events containing either a pair of same-sign $\mu\mu$, $ee$ or $e\mu$ and at least two hadronic jets with a high dijet invariant mass and a large rapidity separation. The largest Standard Model backgrounds to these processes are same-sign $WWjj$ and $WZjj$ production, which are constrained by data in dedicated control regions. Backgrounds from jets misidentified as leptons or from non-prompt decays and electron charge misidentification are modelled with data-driven estimates. A profile likelihood fit is performed to the sub-leading lepton transverse momentum and no significant excess above the Standard Model expectation is observed. Limits are placed at the 95\% confidence interval on the heavy neutrino--light lepton mixing matrix elements $|V_{eN}|^2$, $|V_{\mu N}|^2$ and $|V_{eN} V^*_{\mu N}|^2$ by interpreting the results in the context of the benchmark type-I phenomenological seesaw model. Additionally, the Weinberg operator is considered as an alternative interpretation and upper limits on the effective Majorana neutrino masses $|m_{\mu\mu}|$, $|m_{ee}|$ and $|m_{e\mu}|$ are placed.

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