Dark Photon Searches from Higgs Boson and Heavy Boson Decays Using pp Collisions Recorded at 13 TeV with the ATLAS Detector at the LHC and Performance Evaluation of the Low Gain Avalanche Detectors for the HL-LHC ATLAS High-Granularity Timing Detector

This PhD thesis includes two main topics, the first one is related to the search for dark photons. Two different searches were performed at a centre-of-mass energy of $\sqrt{s}=13$ TeV using the LHC Run-2 integrated luminosity of 139 fb$^{-1}$ recorded by the ATLAS detector, the first search concerns dark photons from the SM Higgs boson decay, produced together with a $Z$ boson, this search resulted in setting the best experimental limit corresponding to an observe (expected) upper limit on the $BR(H\to\gamma\gamma_d )$ of 2.28$\%$ (2.82$\%$) set at 95$\%$ confidence level (CL) for massless $\gamma_d$. To broaden the exploration of this predicted dark matter particle, a second search was carried out considering some heavy Higgs bosons with masses ranging from 400 GeV to 3 TeV, produced either in gluon-gluon Fusion (ggF) or in Vector-Boson-Fusion (VBF) modes. Exclusion limits at 95$\%$ CL on the $\sigma\times BR(H\to\gamma\gamma_d )$ for the combined ggF+VBF productions are found to be in the range from [1.44,24.2] fb ([1.68,28.1] fb) observed (expected) respectively. The second main topic concerns a new timing detector called the High Granularity Timing Detector (HGTD) that will be installed in the ATLAS detector to mitigate pile-up effects during the High Luminosity (HL) run of the LHC. Low Gain Avalanche Detector (LGAD) sensors from different vendors have been measured in beam test campaigns during the years 2018 and 2019 at CERN and DESY. The results obtained in terms of the collected charge, time resolution, hit efficiency as well as the charge uniformity as a function of the position of the incident particle inside the sensor pad are included. In addition, this dissertation includes as an appendix the qualification task project related to the Transition-Radiation-Tracker (TRT) sub-detector of ATLAS. A systematic study was performed to identify sources of discrepancies between the TRTxAOD and the calibration formats. This work was achieved by identifying and fixing the issue in the TRT reconstruction software after being validated in both data simulated samples.