Asymmetric dark matter and effective number of neutrinos

Abstract

We study the effect of the MeV-scale asymmetric dark matter annihilation on the effective number of neutrinos N eff at the epoch of the big bang nucleosynthesis. If the asymmetric dark matter χ couples more strongly to the neutrinos ν than to the photons γ and electrons e − , Γχγ,χe ≪ Γχν , or Γχγ,χe ≫ Γχν, the lower mass limit on the asymmetric dark matter is about 18 MeV for N eff ≃ 3.0.