UC Berkeley

This thesis describes the search for displaced photons in the exotic decays of the Standard Model Higgs boson using 139 fb−1 of proton-proton collision data recorded by the ATLAS detector. The signal model involves the decay of the Higgs boson into a pair of long-lived supersymmetric particles, each of which subsequently decay into a photon and a stable supersymmetric particle. The current upper limits on the decays of the Higgs boson to the undetected particles is around 21% making this search extremely relevant. This analysis utilizes the ATLAS electromagnetic calorimeter’s unique capabilities to measure the photon’s direction of flight and its time of arrival. No significant excess was observed compared to theestimated Standard Model background and this null result was used to set 95% confidence level limits on the branching ratio of the Higgs to the pair of long-lived particles for the very first time.