Detecting Boosted Dark Photons with Gaseous Detectors

We search for indirect signals of $\mathscr{O}$(keV) dark matter annihilating or decaying into $\mathscr{O}$(eV) dark photons. These dark photons will be highly boosted and have decay lengths larger than the Milky Way, and can be absorbed by neutrino or dark matter experiments at a rate dependent on the photon-dark photon kinetic mixing parameter and the optical properties of the experiment. We show that current experiments can not probe new parameter space, but future large-scale gaseous detectors with low backgrounds (i.e. CYGNUS, NEXT, PANDAX-III) may be sensitive to this signal when the annihilation cross section is especially large.