UC Davis

A search for a low mass dark photon below 1 MeV which is radiated from a muon using proton-proton collision data at a center-of-mass energy $\sqrt{s}$ = 13 TeV is presented. The data was collected with the CERN CMS detector in 2018 corresponding to an integrated luminosity of 59.8 fb$^{-1}$. Such a low mass dark photon has no available decay channel to standard model particles, and therefore is stable. The dark photon is assumed to directly interact with detector materials through pair-production, and its small kinetic mixing leads to it depositing energy outside the CMS Electromagnetic Calorimeter. The search is performed by looking for events whose final state is composed of two muons and a photon-like shower in the CMS Hadronic Calorimeter, with the 3-body mass consistent with the Z boson. The accessible range of kinetic mixing in this search is between 0.032 to 0.32. No significant deviation from estimated backgrounds is observed with the 2018 data. The dark photon with kinetic mixing between 0.073 to 0.32 and mass lower than 1 MeV is excluded at 95\% confidence level.