Search for long-lived neutral particles decaying into displaced lepton jets in proton--proton collisions at $\sqrt{s}$ = 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 $\sqrt{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 $\gamma_{\mathrm{d}}$), which in turn produce lepton-jets, are derived as a function of the particle proper decay length, c$\tau$. 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$~\leq~c\tau~\leq~$111.3$~$mm in a Higgs$~\rightarrow~2\gamma_{\mathrm{d}}+X$ benchmark model and in the range 3.8$~\leq~c\tau~\leq~$163$~$mm in a Higgs$~\rightarrow~4\gamma_{\mathrm{d}}+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.