Searching for long-lived particles Beyond the Standard Model with displaced vertex signatures using the ATLAS detector: Not all hopes for new physics die immediately

Traditional LHC searches assume that Beyond the Standard Model particles decay immediately after they are produced. However, there is no a priori motivation for this assumption. This thesis details part of the growing effort to use new unorthodox signatures to search for models where these particles are long-lived. All searches were conducted with the full 139fb$^{−1}$ of Run 2 data taken in 2015-2018. The first analysis uses displaced vertex signatures in multijet triggered events to search for long-lived particles that decay into hadrons in the ATLAS inner detector. This was the first such Run 2 search, placing the best limits in this lifetime range to date on R-parity violating supersymmetry models. The second analysis uses a similar strategy, but using missing transverse energy as the trigger. This improves upon the early Run 2 2016-data search with an updated analysis strategy and a new vertexing algorithm. A focus on interpreting this search with the the Higgs Portal model predicts sensitivity to the exclusion of the Higgs branching ratio to hadronically-decaying long-lived particles to the lowest limit to date. As an outlook to the future of displaced vertex analyses using the ATLAS inner tracker upgrade (ITk), a script library is also described for the automation of quality control for its strip sensors.