Probing the freeze-in mechanism in dark matter models with $U(1)^\prime$ gauge extensions

TL;DR

This paper investigates how MeV-scale dark photons with tiny couplings can produce dark matter via freeze-in, analyzing thermal mixing effects, experimental constraints, and implications for $U(1)_{B-L}$ gauge models.

Contribution

It provides a detailed study of dark photon effects on freeze-in dark matter production, including thermal mixing and experimental constraints, especially for $U(1)_{B-L}$ extensions.

Findings

Dark photons can produce observable signals in fixed-target experiments.

Current XENON1T data constrains GeV-TeV dark matter from freeze-in.

Thermal mixing influences dark matter relic abundance.

Abstract

New gauge bosons at the MeV scale with tiny gauge couplings (so-called dark photons) can be responsible for the freeze-in production of dark matter and provide a clear target for present and future experiments. We study the effects of thermal mixing between dark photons and Standard Model gauge bosons and of the resulting plasmon decays on dark matter production before and after the electroweak phase transition. In the parameter regions preferred by the observed dark matter relic abundance, the dark photon is sufficiently long-lived to be probed with fixed-target experiments and light enough to induce direct detection signals. Indeed, current limits from XENON1T already constrain Dirac fermion dark matter in the GeV to TeV range produced via the freeze-in mechanism. We illustrate our findings for the case of a $U(1)_{B-L}$ gauge extension and discuss possible generalisations.