A Search for Displaced Leptons from Long-Lived Particle Decays with the ATLAS Detector

This thesis presents a search for leptons displaced from the primary vertex with the ATLAS detector at the Large Hadron Collider at CERN. The search includes the full proton--proton collision dataset collected during Run 2 from 2015-2018 at $\sqrt{s}=13$ TeV and a partial dataset collected during Run 3 in 2022-2023 at $\sqrt{s}=13.6$ TeV, corresponding to integrated luminosities of 140 fb$^{-1}$ and 56.3 fb$^{-1}$, respectively. The search is the first performed using proton-proton collision data collected by the ATLAS detector during Run 3, at the highest collision energy ever achieved at a collider. Final states with displaced electrons or muons are considered, and novel triggers introduced in Run 3 are employed that use large impact parameter tracking to reconstruct displaced tracks with low momentum. In addition, multivariate techniques and timing information from the ATLAS electromagnetic calorimeter are employed to broaden the sensitivity to channels with large background rates or highly displaced electrons. The results are consistent with the Standard Model background expectations and are used to set model-independent limits on the production of displaced electrons and muons. The analysis is also interpreted in the context of a gauge-mediated supersymmetry breaking model with pair-produced long-lived sleptons. The results include 95\% CL exclusions of selectrons with lifetimes from 4 ps to 60 ns and a mass of 150 GeV, and exclusions of selectrons, smuons, and staus with a lifetime of 0.3 ns for masses up to 740 GeV, 830 GeV, and 440 GeV, respectively.