Abstract
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 $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$. 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 $1\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{g}$. Severe constraints from the BBN on ${N}_{\mathrm{eff}}$ require that the dark and visible sector are not in thermal equilibrium during freeze-out. The effect of this temperature asymmetry is studied.
Highlights
For the past few decades, we have extensively studied the gravitational interaction of Dark Matter (DM) and very little doubt remains of its existence
One of the simple solutions is to assume that the DM is light i.e., its mass is in the sub-GeV domain
From the model building perspective, it was recently proposed that the 3-to-2 and 4-to-2 annihilations may be important for MeV and keV scale DM respectively [11]
Summary
For the past few decades, we have extensively studied the gravitational interaction of Dark Matter (DM) and very little doubt remains of its existence (for an overview, see [1,2,3,4,5,6] and references therein). Weakly interacting massive particles (WIMPs) were postulated to be DM candidate but recent bounds from null results of terrestrial experiments have ruled out almost all of the interesting parameter space [7]. Several new candidates have been proposed recently which get the correct relic abundance and are consistent with present detector bounds. One of the simple solutions is to assume that the DM is light i.e., its mass is in the sub-GeV domain. In this limit, the local DM cannot produce sufficient recoil and will remain undetected in the traditional detectors. Observation of galaxy cluster collisions puts a strong bound on this self-interaction.
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