Dark sector searches in final states with prompt neutral particles with the ATLAS detector

This work investigates the possibility that dark matter could be produced at the LHC, under the assumption that it is weakly coupled to the ordinary matter via a mixing parameter and that it constitutes a new sector of particles. Under the assumption of a neutral mediator, the dark photon, that decays promptly inside the ATLAS innermost detector volume, the lepton-jet signature, in which collimated leptons are reconstructed as a single object, is exploited. This work, based on the full Run-2 dataset, aims at extending the sensitivity of the previous Run-1 search to smaller branching ratios of the Higgs boson decaying to dark photons and to a wider dark photon mass range in the 10 ?5 region.