Searches for Rare Exclusive Higgs boson decays to a Meson and an Associated Photon with the ATLAS Detector

Searches for the Higgs boson decaying into a meson and an associated photon are presented using $\sqrt{s}=13$ TeV proton-proton collision data collected by the ATLAS experiment during LHC Run II in 2015 and 2016. No significant excess of events is observed above the expected backgrounds. 95% confidence-level upper limits on the branching fractions of the Higgs boson decays to $J/\psi\,\gamma $, $\psi(2S)\,\gamma $, and $\Upsilon(nS)\,\gamma $ of $3.5\times 10^{-4}$, $2.0\times 10^{-3}$, and $(4.9, 5.9, 5.7) \times 10^{-4}$ ($n=1, 2, 3$), respectively, are obtained assuming Standard Model Higgs production with 36.1 fb$^{-1}$ of data. Analogous limits on the branching fractions of the Higgs boson decays to $\phi\,\gamma $ and $\rho\,\gamma $ of $4.8\times 10^{-4}$ and $8.8\times 10^{-4}$, respectively, are also found using up to 35.6 fb$^{-1}$ of data at $\sqrt{s}=13$ TeV. In the next decade, the LHC will undergo an upgrade to increase the delivered instantaneous luminosity, which will aim to collect 3000 fb$^{-1}$ of integrated luminosity over its lifetime. With this improvement the searches for Higgs boson decays into a meson and an associated photon will have a sensitivity closer to the Standard Model predicted branching fractions. The ATLAS detector will be upgraded to sustain performance with the higher rate of collisions. The University of Birmingham is among a consortium of facilities that irradiate and test prototype silicon sensors intended for the upgrade of the ATLAS detector. This thesis also presents early results during the commissioning of the sensor test system.