Search for light long-lived neutral particles from Higgs boson decays via vector-boson-fusion production from $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This paper describes a search for displaced dark photons with masses between 0.1 GeV and 15 GeV, from exotics decays of Higgs bosons produced via vector-boson-fusion. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer to select dark photon candidates that subsequently decay into collimated jets of leptons or light hadrons. This search is based on the full LHC Run 2 (2015–2018) dataset collected in proton-proton collisions at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 139 $\mbox{fb\(^{-1}\)}$. Events that contain displaced collimated Standard Model fermions are reconstructed in the calorimeter or muon spectrometer. Dominant backgrounds from Standard Model processes and non-collision sources are estimated in data-driven techniques. The observed event yields in the signal regions are consistent with the expected background. Upper limits on the production cross-section times branching fraction are reported as a function of the mean 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 dark sectors. Results are also combined with previous ATLAS limits obtained in the gluon-gluon fusion and WH production modes.