Search for long-lived neutral particles decaying into displaced lepton-jets in proton–proton collisions at √s = 13 TeV with the ATLAS detector

This note presents the results of a search for long-lived neutral particles decaying into collimated jets of light leptons and mesons, so-called "lepton-jets", using a sample of 3.4 fb−1 of proton-proton collisions data at a centre-of-mass energy of s√= 13 TeV collected during 2015 with the ATLAS detector at the LHC. No deviations from Standard Model expectations are observed. Limits on models predicting Higgs boson decays to neutral long-lived particles (dark photons γd), which in turn produce lepton-jets, are derived as a function of the particle proper decay length, cτ. Assuming the Standard Model gluon-fusion production cross section for a 125 GeV Higgs boson, its branching ratio to dark photons is found to be below 10%, at 95% CL, for dark-photon lifetimes in the range 2.2 ≤ cτ ≤ 111.3 mm in a Higgs → 2γd+X benchmark model and in the range 3.8 ≤ cτ ≤ 163 mm in a Higgs → 4γd+X model. These limits improve upon the results of a similar search previously performed during Run 1 of the LHC, due to enhancements in the trigger and reconstruction of highly-collimated muons.
ATLAS-CONF-2016-042