Franklin, MelissaFortman, Anne Winifred2023-06-0220232023-06-012023-05Fortman, Anne Winifred. 2023. Searching for heavy, charged, long-lived particles via ionization energy loss and time-of-flight in the ATLAS detector using 140 fb$^{−1}$ of $\sqrt{s}$ = 13 TeV proton-proton collision data. Doctoral dissertation, Harvard University Graduate School of Arts and Sciences.30491669https://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37375754Many extensions to the Standard Model predict the existence of new, massive, long-lived particles. Such heavy particles produced in proton-proton collisions at center-of-mass energy $\sqrt{s}$ = 13 TeV are expected to move significantly slower than the speed of light as they traverse a detector. As a charged particle passes through a material, the amount of energy it loses via ionization is related to its speed. This property can be used to identify slow-moving particles. A heavy, charged, long-lived particle should be identifiable in a detector as a trajectory with high momentum and anomalously large ionization energy loss. The first search for heavy, charged particles using ionization energy loss in the full ATLAS Run 2 dataset observed an excess with a global significance of 3.3$\sigma$ at high mass. We discuss these results and present a followup search for slow-moving particles in 140 fb$^{-1}$ of ATLAS Run 2 data. This new search expands the analysis strategy by incorporating a second measure of particle speed, time-of-flight measurements, to significantly reduce background and improve sensitivity to heavy, charged, long-lived particles. Results are interpreted in terms of supersymmetric models that predict long-lived gluinos, charginos, and sleptons.application/pdfenATLASHigh energy experimentLarge Hadron ColliderParticle physicsThesis or Dissertation2023-06-020000-0001-9454-9069