Light Dark Matter and Dark Radiation

TL;DR

This paper investigates how light dark matter particles, with masses up to 20 MeV, affect the effective number of neutrinos through annihilation processes, using both non-equilibrium and equilibrium models constrained by Planck data.

Contribution

It provides a detailed analysis of non-equilibrium effects of light dark matter annihilation on $N_{eff}$ and compares them with equilibrium approximations, constraining dark matter properties.

Findings

Non-equilibrium effects impose stronger bounds on dark matter mass.

Dark matter annihilation can alter the neutrino-to-photon temperature ratio.

Constraints on additional radiation particles are tightened by non-equilibrium analysis.

Abstract

Light dark-matter ($M\leq20$ MeV) particles freeze out after neutrino decoupling. If the dark-matter particle couples to a neutrino or an electromagnetic plasma, the late time entropy production from dark-matter annihilation can change the neutrino-to-photon temperature ratio, and equally the effective number of neutrinos $N_{eff}$. We study the non-equilibrium effects of dark-matter annihilation on the $N_{eff}$ and the effects by using a thermal equilibrium approximation. Both results are constrained with Planck observations. We demonstrate that the lower bounds of the dark-matter mass and the possibilities of the existence of additional radiation particles are more strongly constrained for dark-matter annihilation process in non-equilibrium.