The Long Game: Searches for Massive, Long-lived Particles at 13.6 TeV with the ATLAS Experiment

Abstract

Long-lived particles (LLPs) travel an observable distance before decaying. They arise naturally in the Standard Model and occur in many Beyond the Standard Model (BSM) theories whenever small couplings, off-shell mediators, or restricted phase space suppress decay rates. The third run of the Large Hadron Collider (Run 3) with the ATLAS detector presents a prime opportunity to probe displaced signatures from BSM LLP decays with improved tracking and triggering capabilities. This thesis presents multiple complementary studies involving displaced particle searches (especially with muons) with the Run 3 ATLAS detector. Chief among these is a new search for massive long-lived particles in events featuring at least one displaced vertex and at least one displaced muon, using proton-proton collision data collected by the ATLAS detector from 2022 to 2024 at $\sqrt{s} = 13.6$ TeV, corresponding to an integrated luminosity of $164$ fb$^{-1}$. Background contributions are estimated using fully data-driven techniques, and no significant excess above the expected background is observed. Upper limits at $95$% confidence level are set on the visible cross-section of one displaced vertex and one displaced muon on the order of $0.01$ fb. World-leading limits are set on the production cross-sections of several lepton-number-violating and baryon-number-violating benchmark models of $R$-parity-violating supersymmetry.