Asymmetric dark matter and effective number of neutrinos

We study the effect of the MeV-scale asymmetric dark matter annihilation on the effective number of neutrinos $N_{\rm eff}$ 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 $e^-$, $Γ_{χγ, χe} \ll Γ_{χν}$, or $Γ_{χγ, χe} \gg Γ_{χν}$, the lower mass limit on the asymmetric dark matter is about $18$ MeV for $N_{\rm eff}\simeq 3.0$.

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