Limits on MeV Dark Matter from the Effective Number of Neutrinos
  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.
