Many extensions of the Standard Model predict a weakly coupled Dark Sector, with the simplest case introducing a U(1) gauge group and a Dark Photon. In the so-called Higgs portal models, Higgs bosons produced in proton-proton collisions at the LHC are allowed to decay into Dark Sector particles, leading to final states with two Dark Photons that decay into collimated Standard Model particles. This work reports a search for Dark Photons with masses from 17 MeV to 20 GeV, promptly decaying into jets of collimated electrons and/or muons (lepton-jets). The analysis uses the full Run-2 dataset of proton-proton collisions at 13 TeV collected by the ATLAS experiment.
