Several extensions of the Standard Model predict the existence of a Dark Sector weakly coupled to the Standard Model via the vector portal. The simplest extension of the Standard Model foresees the addition of a U(1) gauge group and therefore of a new vector boson, the Dark Photon. At the Large Hadron Collider it is also possible to study Higgs portal models, which allows for the Higgs boson to decay into Dark Sector particles. These models results in Higgs decay processes that have two Dark Photons as their final state, which then decay into Standard Model particles, with highly collimated decay products. This work presents the ongoing search for the Dark Photon, with a mass between 17 MeV and 10 GeV, decaying promptly in the ATLAS detector into jets of collimated electrons and/or muons (Lepton-Jets). The analysis is performed by studying the entire Run-2 dataset at a center-of-mass energy of 13 TeV in proton-proton collisions.
