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 $N_{\mathrm{eff}}$ at the epoch of the big bang nucleosynthesis. If the asymmetric dark matter $\chi$ couples more strongly to the neutrinos $\nu$ than to the photons $\gamma$ and electrons $e^{-}$, ${\Gamma }_{\chi \gamma ,\chi e}\ll {\Gamma }_{\chi \nu }$, or ${\Gamma }_{\chi \gamma ,\chi e}\gg {\Gamma }_{\chi \nu }$, the lower mass limit on the asymmetric dark matter is about $18$ MeV for $N_{\mathrm{eff}}\simeq 3.0$.

Comments: 13 pages, 14 figures, version accepted for publication in Physical Review D