Search for exotic decays of the Higgs boson to at least one photon and missing transverse momentum using 79.8 fb$^{-1}$ of proton--proton collisions collected at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for Standard Model Higgs bosons decaying into at least one photon and missing transverse momentum is performed using 79.8 fb$^{-1}$ of proton–proton collisions collected at $\sqrt{s}=13$ TeV with the ATLAS detector, where the photon can arise from a neutralino decaying into a gravitino. 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 near massless gravitinos.