A Search For Long Lived Particles that Decay to Muons Using the ATLAS Detector

Yexley, Melissa and Jones, Roger William Lewis (2023) A Search For Long Lived Particles that Decay to Muons Using the ATLAS Detector. PhD thesis, Lancaster University.

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Abstract

This thesis presents an analysis that uses the ATLAS detector to search for long-lived particles decaying to muons using 139 inverse femtobarns of √s = 13 TeV proton-proton collisions produced by the Large Hadron Collider (LHC) between 2015 and 2018. The analysis specifically searches for a pair of isolated, opposite sign muons whose tracks have an absolute transverse impact parameter, with respect to the centre of the beam spot, between 0.6 mm and 3 mm. This signature has sensitivity to the production of theoretical supersymmetric muon particles with lifetimes between 1 ps and 100 ps. The analysis uses a data driven background estimate. Results are consistent with the background estimate, therefore limits on production are determined in the context of a simplified Gauge Mediated Supersymmetry Breaking (GMSB) model where pair produced smuons each decay to a gravitino and a muon. Left- and right-handed muon super-partners with masses of at least 50 GeV have been excluded at 95% CLs for proper decay times between 1 ps and 120 ps, with a maximum exclusion of 520 GeV occurring at a proper decay time of 10 ps. This is the first study that uses ATLAS to search for opposite sign muon pairs in this transverse impact parameter region that does not require the leptons to come from a common displaced vertex.