Dark Matter Searches in Association with a Higgs Boson using √s = 13 TeV ATLAS Data and Performance Evaluation of the High-Granularity Timing Detector

The overwhelming evidence for the existence of dark maMer in the universe presented by astrophysical and cosmological observations motivates the search program for dark matter candidates at the LHC experiments. If these new particles can be produced in proton-proton collisions at the LHC, they would appear as missing transverse momentum signatures due to their weak interaction with baryonic matter. Additionally, the discovery of a Higgs boson at both ATLAS and CMS in 2012 opened new possibilities to search dark matter candidates produced in association with a Higgs boson. This thesis presents two analyses based on the full ATLAS Run 2 dataset of 139 fb-1 recorded between 2015 and 2018, searching for dark maMer candidates produced in association with a Higgs particle. In both analyses, the Higgs boson is reconstructed exclusively in the diphoton channel. The Mono-Higgs signature is based on a mediator particle, connecting the Standard Model with the dark sector, showing a Higgs boson and dark matter particles in the final state. The search for electroweakino production looks for a Higgs boson, a W boson and missing transverse momentum in the final state. This thesis also contains extensive performance studies performed for the High-Granularity Timing Detector, an ATLAS upgrade project that will be installed in the ATLAS cavern before the start of the High-Luminosity LHC runs in 2027. The high precision timing information of 30 - 50 ps per track provided by this detector opens up new strategies for pile-up mitigation, which are based on a good track-time reconstruction capability. An in depth assessment of first reconstruction algorithms and the posed challenges is provided.