Search for dark photons in rare $Z$ boson decays with the ATLAS detector

A search for the dark photon produced in association with the dark Higgs boson via rare decays of the Standard Model $Z$ boson is presented using 139 $fb^{-1}$ of $\sqrt{s} = 13$ TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. The dark Higgs boson decays into a pair of dark photons and at least two of the three dark photons further decay into pairs of electrons and muons, resulting in at least two same-flavour opposite-charge lepton pairs in the final state. The average invariant mass of the two lepton pairs is used to discriminate signal from the background. The data are found to be consistent with the background prediction and upper limits are set on the dark photon coupling to the dark Higgs boson times the kinetic mixing between the Standard Model photon and the dark photon, $\alpha_{D}\epsilon^2$, in the dark photon mass range of $[5, 40]$ GeV, excluding the $\Upsilon$ mass window $[8.8, 11.1]$ GeV. These limits cover significantly wider mass ranges than previous experiments.