Sub-MeV Dark Sink Dark Matter

TL;DR

This paper explores sub-MeV dark matter models with a Dark Sink mechanism involving dark photons, highlighting plasmon decay as a key energy transfer process and providing benchmarks for direct detection experiments.

Contribution

It extends Dark Sink models to sub-MeV masses, analyzing plasmon decay effects and providing computational benchmarks for experimental searches.

Findings

Plasmon decays dominate energy transfer in sub-MeV dark matter.

Increased dark matter coupling enhances direct detection prospects.

Benchmark ranges for masses and cross sections are derived.

Abstract

A Dark Sink uses dark-sector interactions to siphon energy from dark matter to lighter dark degrees of freedom, i.e. dark radiation. Here, we extend dark matter models containing a Dark Sink to sub-MeV masses. We consider a Dark Sink model where the dark matter is charged under a light dark photon that has kinetic mixing with the Standard Model. For sub-MeV dark matter masses, plasmon decays are the dominant mechanism for transferring energy to the dark sector. Relative to a standard freeze-in cosmology, reproducing the observed dark matter density in a Dark Sink structure requires an increase in the dark matter couplings to the Standard Model, and hence increased direct detection cross sections. These models provide benchmarks for current and upcoming direct detection experiments. Accounting for plasmon effects, we derive the range of possible dark matter masses and cross sections for Dark Sink models in the sub-MeV regime. We make code available to reproduce our benchmarks; it may be of use for other freeze-in scenarios, including those where plasmon decays to the dark matter are important.