Possible s-wave annihilation for MeV dark matter with the 21-cm
  absorption
  The CMB observation sets stringent constraints on MeV dark matter (DM)
annihilating into charged states/photons in s-wave, and the recent observation
of the 21-cm absorption at the cosmic dawn reported by EDGES is also very
strict for s-wave annihilations of MeV DM. The millicharged DM with p-wave
dominant annihilations during the freeze-out period are considered in
literatures to give an explanation about the 21-cm absorption, with photon
mediated scattering cooling the hydrogen. In this paper, we focus on the
annihilation of millicharged DM being s-wave dominant. To explain the 21-cm
absorption and meanwhile be compatible with the CMB and 21-cm absorption bounds
on DM annihilations, we consider the annihilation close to the resonance, with
the new mediator (here is dark photon) mass being slightly above twice of the
millicharged DM mass. In this case, the annihilation cross section at the
temperature $T \to 0$ could be much smaller than that at $T_f$, which would be
tolerated by the bounds on DM annihilations, avoiding the excess heating from
DM s-wave annihilations to the hydrogen gas. The beam dump and lepton collider
experiments can be employed to hunt for millicharged DM via the production of
the invisible dark photon.
