http://arxiv.org/abs/1910.05633
A small fraction of millicharged dark matter (DM) is considered in the literature to give an interpretation about the enhanced 21-cm absorption at the cosmic dawn, while the main component of DM is still unclear. Here we focus on the case that the main component is self-interacting dark matter (SIDM), motivated by the small scale problems. For self interactions of SIDM being compatible from dwarf to cluster scales, Sommerfeld enhanced velocity-dependent self interactions mediated by a light scalar $\phi$ is considered. To fermionic SIDM $\Psi$, the main annihilation mode $\Psi \bar{\Psi} \to \phi \phi$ is a $p -$wave process, which could evade constraints from CMB and indirect detections. For thermal freeze-out type SIDM, the thermal equilibrium between SIDM and the standard model (SM) particles in the early universe via the transition of SIDM $\rightleftarrows \phi \rightleftarrows$ SM sets a lower bound on couplings of $\phi$ to SM particles, which has been excluded by DM direct detections, and here we consider SIDM in the thermal equilibrium with millicharged DM. For $m_\phi >$ twice millicharged DM mass, $\phi$’s lifetime could be much smaller than 1 second, avoiding excess energy injection to the big bang nucleosynthesis. Thus, the $\phi -$SM particle couplings could be very tiny and evade DM direct detections. The momentum transfer in SIDM-target nucleus scattering may be comparable with $m_\phi$ in direct detections, and the picture of WIMP-nucleus scattering with contact interactions fails for SIDM-nucleus scattering with a light mediator. A method is explored in this paper, with which a WIMP search result can be converted into the hunt for SIDM in direct detections.
L. Jia
Tue, 15 Oct 19
86/90
Comments: 11 pages, 7 figures
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