# Thermal Dark Matter Below an MeV

## TL;DR

This paper explores models where thermal dark matter below 1 MeV can thermalize with the Standard Model after neutrino-photon decoupling, affecting cosmological constraints and opening new experimental avenues.

## Contribution

It introduces a model with fermionic dark matter and a light scalar mediator that allows sub-MeV thermal dark matter consistent with observations.

## Key findings

- Dark matter can be as light as a few keV.
- Thermalization below neutrino decoupling alters cosmological constraints.
- Supports new experiments for sub-MeV dark matter detection.

## Abstract

We consider a class of models in which thermal dark matter is lighter than an MeV. If dark matter thermalizes with the Standard Model below the temperature of neutrino-photon decoupling, equilibration and freeze-out cools and heats the Standard Model bath comparably, alleviating constraints from measurements of the effective number of neutrino species. We demonstrate this mechanism in a model consisting of fermionic dark matter coupled to a light scalar mediator. Thermal dark matter can be as light as a few keV, while remaining compatible with existing cosmological and astrophysical observations. This framework motivates new experiments in the direct search for sub-MeV thermal dark matter and light force carriers.