Search for Displaced Photons from Exotic Decays of the Higgs Boson with the ATLAS Detector

A search for displaced photons from exotic decays of the Higgs boson is presented, specifically targeting the relatively weak constraints on the branching ratio of Higgs boson decays to invisible particles imposed by other searches. 139 fb⁻¹ of 𝑝-𝑝 collision data at center-of-mass energy √𝑠 = 13 TeV, collected between 2015 and 2018, is analyzed. Exploiting the excellent timing performance and longitudinal segmentation of the ATLAS electromagnetic calorimeter, the photon arrival time and extrapolation to the beam axis, or pointing, can be used to extract the displaced photon signature from Standard Model background processes. The background photon timing and pointing distributions are predicted using carefully constructed, data-derived templates. Signal contributions are analyzed in the context of a Gauge-Mediated Supersymmetry Breaking model in which the Higgs boson decays into two, long-lived next-to-lightest supersymmetric particle neutralinos, which each subsequently decay into a photon and a gravitino. Agreement with background is observed with no significant excesses for any signal models considered. Limits at the 95% confidence level are placed on the branching ratio of the Higgs boson to two neutralinos in the context of the various signal models.