Sub-MeV self-interacting dark matter

In this paper, we present a model for sub-MeV dark matter with strong self-interactions which can solve some of the small-scale crisis of the ΛCDM. The dark matter is a Majorana fermion with only off-diagonal interactions with a hidden U(1)D gauge boson. The relic density is obtained by freeze-out of Boltzmann suppressed annihilations to a light fermionic species. The self-interaction is a one-loop process and constrained to be between 0.1 to 1 cm2/g. Severe constraints from the BBN on Neff require that the dark and visible sector are not in thermal equilibrium during freeze-out. The effect of this temperature asymmetry is studied.