Search for dark-photons decaying to lepton-jets with the ATLAS detector at LHC

Several new physics models predict the existence of neutral particles with macroscopic life-times that decay to pairs of leptons and light hadrons with a jet-like structure (lepton-jets). These particles, decaying outside of the interaction region, will give rise to striking signatures in the detectors at the LHC. These can be detected through numerous unconventional signatures: long time-of-flight, late calorimetric energy deposits or displaced vertices. The most recent ATLAS results based on 36\ifb of data collected in proton-proton collisions at \sqrt(s)=13TeV recorded in 2015-2016 are presented. Prospects for the search for these particles with the ATLAS detector upgrade at HL-LHC are also given.