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 $$\phi $$ ϕ are considered. For fermionic SIDM $$\Psi $$ Ψ, the main annihilation mode $$\Psi \bar{\Psi } \rightarrow \phi \phi $$ Ψ Ψ ¯ → ϕ ϕ is a p -wave process. The thermal transition of SIDM $$\rightleftarrows \phi \rightleftarrows $$ ⇄ ϕ ⇄ standard model (SM) particles in the early universe sets a lower bound on couplings of $$\phi $$ ϕ to SM particles, which has been excluded by direct detections of DM, and here we consider SIDM in thermal equilibrium via millicharged DM. For $$m_\phi>$$ m ϕ > twice millicharged DM mass, $$\phi $$ ϕ could decay quickly and avoid excess energy injection to big bang nucleosynthesis. Thus, the $$\phi $$ ϕ –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.

Authors

Lian-Bao Jia

Published

Feb 1, 2020

Journal or venue

The European Physical Journal C

DOI

10.1140/epjc/s10052-020-7656-9

Paper type

journal article

Source

Crossref