Seeing the Light (Higgs): Searches and Measurements of Higgs Boson Decays to Photons

This dissertation presents two results involving Standard Model Higgs bosons decaying to at least one photon. These analyses are performed using $79.8^{-1}$ of proton--proton collisions collected at $\sqrt{s} = 13$ TeV with the ATLAS detector at the Large Hadron Collider. Differential and fiducial cross section measurements of the Higgs boson decaying to two photons are presented. The inclusive diphoton cross section was measured to be $55 \pm 9 \, \text{(stat.)} \pm 4 \, \text{(syst.)} \pm 0.1 \, \text{(theo.)} \, \text{fb}$ compared with the Standard model prediction of $64 \pm 2 \, \text{fb}$. In addition to inclusive cross section measurement, fiducial cross section results are shown for regions targeting the vector boson fusion, VH, and ttH production modes of the Higgs boson as well as a region targeting beyond the Standard Model contributions. Differential results are given for multiple measured spectra involving kinematics of the diphoton system or variables with jets produced in association with the Higgs boson. No significant deviations from the Standard Model are observed and in their absence, limits are set on alternative hypotheses involving higher order couplings between Standard Model particles and the Higgs boson. Additionally, a search for a Standard Model Higgs bosons decaying into at least one photon and missing transverse momentum was performed. The photon can arise from a Higgs boson decaying into one or two neutralinos, which in turn decay into a gravitino and photon. Higgs bosons produced in association with a Z boson are considered in order to reduce the number of background events. No excess with respect to the Standard Model prediction is observed. Assuming a Standard Model ZH production cross section, the branching fraction of a Higgs boson to neutralinos or neutralino/gravitino is constrained to be less than $5$-$11\%$ at the $95\%$ confidence level for nearly massless gravitinos.