Limits on MeV Dark Matter from the Effective Number of Neutrinos

TL;DR

This paper derives lower bounds on MeV-scale dark matter mass based on its effects on the effective number of neutrinos, N_eff, using cosmological observations and particle physics considerations.

Contribution

It provides new constraints on MeV dark matter mass by analyzing its impact on N_eff and neutrino decoupling, improving previous bounds especially for p-wave annihilation.

Findings

Dark matter mass must be greater than 3-9 MeV.

Constraints are stronger than those from CMB fluctuation distortions for p-wave annihilation.

Dark matter interactions influence the neutrino-electron-photon thermal history.

Abstract

Thermal dark matter that couples more strongly to electrons and photons than to neutrinos will heat the electron-photon plasma relative to the neutrino background if it becomes nonrelativistic after the neutrinos decouple from the thermal background. This results in a reduction in N_eff below the standard-model value, a result strongly disfavored by current CMB observations. Taking conservative lower bounds on N_eff and on the decoupling temperature of the neutrinos, we derive a bound on the dark matter particle mass of m_\chi > 3-9 MeV, depending on the spin and statistics of the particle. For p-wave annihilation, our limit on the dark matter particle mass is stronger than the limit derived from distortions to the CMB fluctuation spectrum produced by annihilations near the epoch of recombination.