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Light Dark Matter and Dark Radiation

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.