Search for a dark photon in resonant mono-photon signatures from Higgs boson decays

The search for dark photon (y_d) in resonant mono-photon signatures from the Higgs boson decay H->yy_d in the ZH production mode with Z->ll has been performed using 139 fb-1 of proton???proton collision data recorded with the ATLAS detector at a centre-of-mass energy sqrt(s) = 13 TeV at the Large Hadron Collider during the 2015-2018 Run 2. A global fit to the Boosted Decision Tree (BDT) score, including all background processes, has been performed to estimate the excess of events that could be interpreted as a possible signature of H->yy_d in the final state. Dominant backgrounds, consisting in fake Etmiss and e->y, have been estimated using data-driven techniques: an ABCD method has been implemented for fake Etmiss while e->y fake-rates have been estimated via Z->ee boson decay and applied in dedicated probe-e CRs. Results covering a y_d mass range from massless up to 40 GeV are provided in terms of limits on the branching ratio of such a decay mode of the Higgs boson.