Search for Long-Lived Dark Photons from Dark Radiation at the LHC

Abstract

We investigate a novel production mechanism for long-lived dark photons at the LHC, arising from dark radiation emitted from $\chi$ in $Z\to\bar{\chi}\chi$ decays, where $\chi$ is a fermionic dark matter candidate. The effective $Z\chi\chi$ coupling is generated radiatively through one-loop diagrams involving the top quark and a new colored scalar. We show that dark photons produced via this dark radiation channel can dominate over the conventional sources-meson decays and proton bremsstrahlung-across wide regions of parameter space, particularly for small kinetic mixing and dark photon masses well above the GeV scale. Using this enhanced production mechanism, we analyze the sensitivity of dedicated long-lived particle detectors, including FASER2, FACET, and MATHUSLA. We find that these experiments can significantly surpass existing bounds, probing regions of dark photon parameter space consistent with the observed dark matter relic abundance and inaccessible in conventional dark photon scenarios.