Search for dark gauge bosons decaying into displaced lepton-jets in proton–proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

The dark photon (A'), the gauge boson carrier of a hypothetical new force, has been proposed in a wide range of Beyond the Standard Model (BSM) theories, and could serve as our window to an entire dark sector. A massive A' could decay back to the Standard Model (SM) with a significant branching fraction, through kinetic mixing with the SM photon. If this A' can be produced from decays of a dark scalar that mixes with the SM Higgs boson, collider searches involving leptonic final states provide promising discovery prospects with rich phenomenology. This work presents the results of a search for dark photons in the mass range $0.2 \lesssim m_{A'} \lesssim 10$ GeV decaying into collimated jets of light leptons and mesons, so-called "lepton-jets". It employs 3.57 fb ${^-1}$ of data from proton-proton collisions at a centre-of-mass energy of √s= 13 TeV, collected during 2015 with the ATLAS detector at the LHC. No deviations from SM expectations are observed. Limits on benchmark models predicting Higgs boson decays to A′ s are derived as a function of the A′ lifetime; limits are also established in the parameter space of mA′ vs. kinetic mixing parameter $\epsilon$. These extend the limits obtained in a similar search previously performed during Run 1 of the LHC, to include dark photon masses $2 \lesssim m_{A'} \lesssim 10$ GeV and to cover higher $\epsilon$ values for $0.2 \lesssim m_{A'} \lesssim 2$ GeV, and are complementary to various other ATLAS A′ searches. As data-taking continues at the LHC, the reach of lepton-jet analyses will continue to expand in model coverage and in parameter space.