Direct Detection and Cosmological Constraints of Dark Matter with Dark Dipoles
We study a fermionic dark matter candidate that couples to the standard model particles exclusively through electric and magnetic dipole operators mediated by a massive dark photon. Such dipole portals naturally arise in dark sectors where the dark matter is neutral under a hidden $U(1)_D$, and they lead to phenomenology distinct from conventional vector-current interactions. We consider the direct-detection signals arising from dark matter-nucleus scattering including the Migdal effect, dark matter-electron scattering, and semiconductor targets, which allow sensitivity to sub-GeV dark matter masses, together with the cosmological bounds from such as thermal relic abundance, cosmic microwave background, big-bang nucleosynthesis, and cosmic-rays. We find that the dark dipole coupling can be largely constrained by direct detection (in particular, electric dipole coupling). However, the cosmological observations have already constrained most of the parameter space, in particular for magnetic dipole interactions of $U(1)_D$ for sub-GeV dark matter. For the dark matter mass below 10 MeV, the semiconductor (in particular, using skipper-CCD) experiments can play a crucial role in probing the dark dipole interactions: future low-threshold experiments utilizing the semiconductor targets can further extend the constraints. Our results have demonstrated that the sub-GeV dark matter with dark dipole interactions can be still safe from the direct-detection constraints, and the future low-threshold semiconductor experiments may play a significant role in constraining the dark dipole interactions.