Light(ly)-coupled Dark Matter in the keV Range: Freeze-In and Constraints

TL;DR

This paper explores keV-range dark matter produced via freeze-in mechanisms, analyzing constraints from stellar cooling and potential detection prospects, highlighting the parameter space where such dark matter could exist.

Contribution

It introduces new constraints on light dark matter produced by freeze-in, considering specific couplings and astrophysical processes, and discusses the detectability in upcoming experiments.

Findings

Dark matter below tens of keV is excluded by stellar cooling constraints.

Freeze-in production for keV-scale dark matter is viable only above certain mass thresholds.

Upcoming direct detection experiments may probe some parameter space for dipole-moment dark matter.

Abstract

Dark matter produced from thermal freeze-out is typically restricted to have masses above roughly 1 MeV. However, if the couplings are small, the freeze-in mechanism allows for production of dark matter down to keV masses. We consider dark matter coupled to a dark photon that mixes with the photon and dark matter coupled to photons through an electric or magnetic dipole moment. We discuss contributions to the freeze-in production of such dark matter particles from standard model fermion-antifermion annihilation and plasmon decay. We also derive constraints on such dark matter from the cooling of red giant stars and horizontal branch stars, carefully evaluating the thermal processes as well as the bremsstrahlung process that dominates for masses above the plasma frequency. We find that the parameters needed to obtain the observed relic abundance from freeze-in are excluded below a few tens of keV, depending on the value of the dark gauge coupling constant for the dark photon portal model, and below a few keV, depending on the reheating temperature for dark matter with an electric or magnetic dipole moment. While laboratory probes are unlikely to probe these freeze-in scenarios in general, we show that for dark matter with an electric or magnetic dipole moment and for dark matter masses above the reheating temperature, the couplings needed for freeze-in to produce the observed relic abundance can be probed partially by upcoming direct-detection experiments.