Millicharged Relics Reveal Massless Dark Photons

TL;DR

This paper explores how millicharged dark relics can enable the detection of massless dark photons through early universe oscillations, offering new avenues for experimental tests and cosmological insights.

Contribution

It introduces a novel mechanism where millicharged particles induce effective dark photon masses, facilitating detection and relaxing cosmological constraints on dark photons.

Findings

Millicharged relics induce photon-dark photon oscillations.

Potential to relax cosmological constraints on dark photons.

Proposes new experimental and observational tests.

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.