Dark-Matter Electric and Magnetic Dipole Moments

TL;DR

This paper investigates neutral dark-matter candidates endowed with nonzero electric or magnetic dipole moments. By combining an effective Lagrangian with relic-abundance calculations, precision-parameter constraints, direct-detection limits, cosmological perturbation theory, and gamma-ray signatures, it maps the viable parameter space. The authors find that a Dirac DM particle with a dipole moment around ~10^{-17} e cm and mass in the MeV–GeV range can satisfy all existing constraints, while larger moments or heavier masses are progressively disfavored; upcoming gamma-ray (GLAST) and direct-detection experiments could probe remaining regions. The work highlights the interplay between particle physics, cosmology, and astrophysical observations in testing nonstandard DM couplings to photons.

Abstract

We consider the consequences of a neutral dark-matter particle with a nonzero electric and/or magnetic dipole moment. Theoretical constraints, as well as constraints from direct searches, precision tests of the standard model, the cosmic microwave background and matter power spectra, and cosmic gamma rays, are included. We find that a relatively light particle with mass between an MeV and a few GeV and an electric or magnetic dipole as large as $\sim 3\times 10^{-16}e$ cm (roughly $1.6\times10^{-5} μ_B$) satisfies all experimental and observational bounds. Some of the remaining parameter space may be probed with forthcoming more sensitive direct searches and with the Gamma-Ray Large Area Space Telescope.

Figures (13)

• Figure 1: The constraints on the dipolar-dark-matter parameter space $[m_\chi,\,({\cal D,\,M})]$ that come from present-day searches and experiments. Viable candidates must lie in the shaded region, below the solid lines and outside the long-dashed lines. The short-dashed "relic abundance" curve shows where the dark matter would have a cosmological density $\Omega_\chi h^2=0.135$, assuming standard freezeout, no particle-antiparticle asymmetry, and no interactions with standard-model particles apart from the dipole coupling to photons.
• Figure 2: Feynman diagrams for annihilation of a DDM--anti-DDM pair to two photons.
• Figure 3: Feynman diagram for DDM--anti-DDM annihilation to fermion-antifermion pairs.
• Figure 4: Feynman diagrams for elastic scattering of an electron from a DDM particle.
• Figure 5: One-loop correction to the photon self-energy induced by dipole moments $\cal{M},\cal{D}$ of the dark-matter particle.