Direct Detection and Cosmological Constraints of Dark Matter with Dark Dipoles

TL;DR

We address a fermionic DM candidate that communicates with the SM exclusively through electric and magnetic dipole interactions mediated by a massive dark photon. The approach combines direct-detection channels (Migdal effect, DM–electron scattering, and semiconductor targets) with cosmological bounds to delineate viable parameter space, finding that sub-GeV dipole-coupled DM can evade current limits, especially in asymmetric scenarios with a light dark photon. The work identifies DarkSide-50 Migdal and future low-threshold semiconductor experiments as crucial probes for MeV–GeV DM and highlights the complementary role of collider and $N_{ ext{eff}}$ constraints on the dark-photon portal. Overall, the results indicate that sub-GeV DM with dark dipole interactions remains viable, with upcoming solid-state detectors poised to substantially tighten the constraints.

Abstract

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.

Figures (2)

• Figure 1: Constraints on the dark dipole moments with $\epsilon = 10^{-3}$: the electric dipole $d_{\chi}$ (top) and the magnetic dipole $\mu_{\chi}$ (bottom). The left column corresponds to $m_{A'} = 3m_{\chi}$, where $\chi$ is the lightest dark-sector particle. Meanwhile, the right column corresponds to $m_{A'} = 0.1m_{\chi}$, where $A'$ is the lightest dark-sector particle. Constraints from the direct-detection experiments include the Migdal effect (DarkSide-50: bright red shaded), DM-electron scattering (XENON10: pink shaded, XENONnT: purple shaded), and semiconductor detector (SENSEI: cyan shaded, DAMIC-M: brown shaded, projected germanium detector: dashed line). Cosmological and astrophysical constraints include the relic abundance (steel blue shaded), the CMB (orange shaded), the BBN (green shaded), and cosmic-rays (gray shaded): these bounds can be easily avoided in the case of asymmetric DM, so we put them in relatively transparent colors and dashed boundaries in the right panels.
• Figure 2: Constraints on the dark photon kinetic-mixing parameter $\epsilon$ as a function of the dark photon mass $m_{A'}$ for the case $m_{A'} = 0.1 m_{\chi}$ and $\mu_\chi = e/2m_\chi$. The color codes for the constraints are the same as in fig:constraints_dipole. Direct-detection limits arise from the Migdal effect (DarkSide-50: bright red), DM-electron scattering (XENON10: pink shaded, XENONnT: purple), and skipper-CCD (SENSEI: cyan), while projected sensitivities from germanium semiconductor detectors are shown as a black-dashed line. Cosmological constraints include limits from $N_\mathrm{eff}$ (freeze-out scenario, light pink), relic abundance (steel-blue), BBN (green), CMB (orange) and cosmic-ray (gray). Again, the constraints on the annihilation processes are shown with relatively transparent colors and dashed boundaries, indicating that these bounds may be relaxed or absent in asymmetric DM scenarios. Collider constraints from BaBar (dark-red) and KLOE (teal), fixed-target constraints from APEX (olive) and SN1987 A constraints (dark-gray) are also included. Meanwhile, in terms of the visibility of the direct-detection constraints, we do not show the constraints from the long-lived particle searches from proton beam-dump and FASER, which are overlapped with the constraints from XENONnT and XENON10.