Franklin, MelissaDiPetrillo, Karri Folan2019-12-122019-052019-05-172019DiPetrillo, Karri Folan. 2019. Search for long-lived, massive particles in events with a displaced vertex and a displaced muon using sqrt{s} = 13 TeV pp-collisions with the ATLAS detector. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.http://nrs.harvard.edu/urn-3:HUL.InstRepos:42029616A search for long-lived particles giving rise to displaced decays with at least one muon is presented. The analysis selects events that pass a muon or missing transverse energy trigger and contain a displaced muon and a displaced vertex with at least three tracks and a vertex visible invariant mass of at least 20~GeV. A data sample of $\sqrt{s} = 13$~TeV proton--proton collisions collected by the ATLAS detector between 2016 and 2018, corresponding to a total integrated luminosity of 136~fb$^{-1}$, is used for this search. The analysis employs dedicated reconstruction techniques that significantly increase the sensitivity to displaced decays. Background estimates for Standard Model processes and instrumental effects are extracted entirely from data. The observed event yields are compatible with those expected from background processes. Results are presented as limits at 95\% confidence level on the model-independent cross section for processes beyond the Standard Model, and interpreted as exclusion limits on scenarios with pair-production of long-lived stop squarks that decay via a small $R$-parity violating coupling to a quark and a muon. Stop squarks with a lifetime of 0.1 ns in these scenarios are excluded for masses up to 1.7 TeV.application/pdfenCERNATLASHigh Energy PhysicsParticle PhysicsSupersymmetryLong-lived particlesThesis or Dissertation2019-12-12