Search for long-lived dark photons through lepton-jets

In recent years at the Large Hadron Collider (LHC), new strategies have appeared to search for new physics related to Dark Matter (DM) by studying exotic long-lived particles (LLP) that would leave distinguishable signatures in detectors. In this context, since the next data-taking period (Run-3) is expected to reach an integrated luminosity no more than two times the one achieved in the last period, many new LLP searches have appeared. Dark/Hidden sector models, which propose a dark sector that is very weakly coupled to known particles, stand out offering particulary interesting signatures. This talk will cover searches for long-lived dark photons mediated by a Higgs boson that decays into displaced collimated fermions with the ATLAS detector. I will give a summary of the latest ATLAS results using 2015-2016 event statistics, follow by the new publication strategy, concentrating in the new vector-boson-fusion (VBF) machine learning classification.