Search for dark matter in events with missing transverse momentum and a Higgs boson decaying to two photons in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for dark matter particles in events with large missing transverse momentum and a Higgs boson candidate decaying to two photons is reported. The search uses $139 \, $fb$^{-1}$ of proton-proton collision data collected at $\sqrt{s}=13$ TeV with the ATLAS detector at the CERN LHC between the years 2015 and 2018. No significant excess of events is observed over the Standard Model predictions, and the results are interpreted extracting limits on three simplified models that include either vector or pseudo-scalar mediators and predict a final state with a pair of dark matter candidates and a Higgs boson subsequently decaying into a photon pair.