Search for light long-lived neutral particles that decay to collimated pairs of leptons or light hadrons in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for light neutral long-lived particles with masses in the \textit{O}(MeV--GeV) range is presented. The analysis targets the production of long-lived dark photons in the decay of a Higgs boson produced via gluon fusion, or in association with a $W$ boson. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer are selected in 139 fb$^{-1}$ of $\sqrt{s} = 13$~TeV $pp$ collision data collected by the ATLAS detector at the LHC. Background estimates for contributions from Standard Model processes and instrumental effects are extracted from data. The observed event yields are consistent with the expected background. Exclusion limits are reported on the production cross section times branching fraction as a function of the proper decay length of the dark photon, or as a function of the dark photon mass and kinetic mixing parameter that quantifies the coupling between the Standard Model and potential hidden (dark) sectors. A Higgs boson branching fraction above 0.1\% is excluded at 95\% CL for a Higgs boson decaying into two dark photons for dark-photon mean proper lifetimes $c\tau$ between 10~mm and 250~mm and dark photons with masses between 0.4~\GeV\ and 2~\GeV.