Through the looking glass and what ATLAS found there: a Dark Sector search for light Dark Matter

Latest theories beyond the Standard Model predict a new `dark force' mediated by a light neutral particle called a dark photon, which opens a window to a complex Dark Sector. Through kinetic mixing a dark photon produced from the decay of a Higgs boson can decay back to SM particles with a sizeable lifetime, giving rise to striking signatures at hadron colliders. This work presents the results of a search for long-lived dark photons decaying into displaced collimated jet-like structures of leptons and light hadrons, referred to as `dark-photon jets'. The search uses data corresponding to an integrated luminosity of intlumi collected in proton--proton collisions at $sqrt{s} = 13$~TeV recorded in 2015--2016 of Run-2 data taking with the ATLAS detector at the Large Hadron Collider. The observed number of events is consistent with the expected background, and limits on the production cross section times branching fraction as a function of the proper decay length of the dark photon are reported. The enormous amount of data that will be collected by ATLAS during the Run-3 ($300~ifb$) and High-Luminosity ($3000~ifb$) 14 ev~LHC phase, and the updated ATLAS detector setup, will offer a unique opportunity to probe unexplored regions of phase space in the context of this search. Sensitivity prospects for Run-3 and High-Luminosity LHC are discussed and two new muon trigger algorithms are studied to improve the selection efficiency of displaced muon pairs. The current dark-photon jet analysis reach will continue to expand in parameter space and signature topologies proving to be a powerful tool for probing the Dark Sector at the LHC.