Millicharged relics reveal massless dark photons
Asher Berlin (Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY, 10003, USA, Theoretical Physics Department, Fermilab, Batavia, IL, 60510, USA); Jeff Dror (Department of Physics, University of California Santa Cruz, and Santa Cruz Institute for Particle Physics, 1156 High St., Santa Cruz, CA, 95064, USA); Xucheng Gan (Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY, 10003, USA); Joshua Ruderman (Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY, 10003, USA)
The detection of massless kinetically-mixed dark photons is notoriously difficult, as the effect of this mixing can be removed by a field redefinition in vacuum. In this work, we study the prospect of detecting massless dark photons in the presence of a cosmic relic directly charged under this dark electromagnetism. Such millicharged particles, in the form of dark matter or dark radiation, generate an effective dark photon mass that drives photon-to-dark photon oscillations in the early universe. We also study the prospect for such models to alleviate existing cosmological constraints on massive dark photons, enlarging the motivation for direct tests of this parameter space using precision terrestrial probes.
