20797 2211.05139 10.1007/JHEP05(2023)046 46 46 Millicharged relics reveal massless dark photons 02 Physical Sciences 0201 Astronomical and Space Sciences Regular Article - Theoretical Physics 1 33 2023 5 5 2022 11 21 2023 4 12 2023 4 28 2023 5 5 The Author(s) 2023 Open Access. This article is distributed under the terms of the Creative Commons Attribution License ( CC-BY 4.0 ), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. 13130 2023 2023 5 5 Asher Berlin aberlin@fnal.gov Jeff A. Dror jdror1@ucsc.edu Xucheng Gan xg767@nyu.edu Joshua T. Ruderman ruderman@nyu.edu grid.137628.9 0000 0004 1936 8753 Center for Cosmology and Particle Physics, Department of Physics New York University New York NY 10003 USA grid.417851.e 0000 0001 0675 0679 Theoretical Physics Department, Fermilab P.O. Box 500 Batavia IL 60510 USA grid.205975.c 0000 0001 0740 6917 Department of Physics, University of California Santa Cruz, and Santa Cruz Institute for Particle Physics 1156 High St. Santa Cruz CA 95064 USA Abstract 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. Keywords Cosmology of Theories BSM Early Universe Particle Physics Particle Nature of Dark Matter ArXiv ePrint: 2211.05139