Search for long-lived neutral particles decaying into Lepton-Jets with the ATLAS detector in proton-proton collision data at 13 TeV

Several models of particle physics different from the Standard Model predict the existence of a dark sector that is weakly coupled to the visible one: the two sectors may couple via the vector portal, where a dark photon with mass in the MeV to GeV range mixes kinetically with the SM photon. If the dark photon is the lightest state in the dark sector, it will decay to SM particles, mainly to leptons and possibly light mesons. Due to its weak interactions with the SM, it can have a non-negligible lifetime. At the LHC, these dark photons would typically be produced with large boost resulting in collimated jet-like structures containing pairs of leptons and/or light hadrons, the so-called Lepton-Jets. This work focuses on the search for displaced Lepton-Jets, which are produced away from the interaction point and their constituents are limited to electrons, muons, and pions. Results from the analysis of 3.4 fb-1 proton-proton collision data sample recorded by ATLAS at a center-of-mass energy of 13 TeV during 2015 are compared to the Standard Model expectations and with BSM predictions.