Velocity-dependent self-interacting dark matter from thermal freeze-out and tests in direct detections

Abstract

A small fraction of millicharged dark matter (DM) is considered in the literature to give an interpretation of the enhanced 21-cm absorption at the cosmic dawn. Here we focus on the case that the main component of DM is self-interacting dark matter (SIDM), motivated by the small-scale problems. For self-interactions of SIDM being compatible from dwarf to cluster scales, velocity-dependent self-interactions mediated by a light scalar phi are considered. For fermionic SIDM Psi , the main annihilation mode Psi bar{Psi } rightarrow phi phi is a p-wave process. The thermal transition of SIDM rightleftarrows phi rightleftarrows standard model (SM) particles in the early universe sets a lower bound on couplings of phi to SM particles, which has been excluded by direct detections of DM, and here we consider SIDM in thermal equilibrium via millicharged DM. For m_phi> twice millicharged DM mass, phi could decay quickly and avoid excess energy injection to big bang nucleosynthesis. Thus, the phi –SM particle couplings could be very tiny and evade direct detections of DM. The picture of weakly interacting massive particle (WIMP)–nucleus scattering with contact interactions fails for SIDM–nucleus scattering with a light mediator, and a method is explored in this paper with which a WIMP search result can be converted into the hunt for SIDM in direct detections.