Dark photon portal dark matter with the 21-cm anomaly
  A strong absorption profile was reported by the EDGES Collaboration, which
indicates the hydrogen gas being colder than expected. It could be signatures
of non-gravitational interactions between normal matter and dark matter (DM),
and a potential explanation is that a small fraction of millicharged DM
scatters with normal matter, with the DM mass in tens of MeV. To obtain the
small fraction of millicharged DM and meanwhile being tolerant with by the
constraints, the dark photon portal scalar and vector millicharged DM are
explored in this paper. We consider that the mass of dark photon is slightly
above twice of the millicharged DM mass, and thus the millicharged DM
predominantly annihilates in p-wave during the freeze-out period, with the
annihilation being enhanced near the resonance. The dark photon mainly decays
into millicharged DM, and couplings of dark photon with SM particles could be
allowed by the lepton collision experiments. The corresponding parameter spaces
are derived. Future lepton collision experiments can be employed to search for
millicharged DM via the production of the invisible dark photon.
