How dark are Majorana WIMPs? Signals from magnetic inelastic dark matter and Rayleigh dark matter

The effective interactions of dark matter with photons are fairly restricted. Yet both direct detection as well as monochromatic γ-ray signatures depend sensitively on the presence of such interactions. For a Dirac fermion, electromagnetic dipoles are possible but are very constrained. For Majorana fermions, no such terms are allowed. We consider signals of an effective theory with a Majorana dark matter particle and its couplings to photons. In the presence of a nearby excited state, there is the possibility of a magnetic dipole transition (magnetic inelastic dark matter), which yields both direct and indirect detection signals and, intriguingly, yields essentially the same size over a wide range of dipole strengths. Absent an excited state, the leading interaction of weakly interacting massive particles is similar to the Rayleigh scattering of low-energy photons from neutral atoms, which may be captured by an effective operator of dimension 7 of the form χ̄χF _μνFμν. While it can be thought of as a phase of the magnetic inelastic dark matter scenario where the excited state is much heavier than the ground state, it can arise from other theories as well. We study the resulting phenomenology of this scenario: gamma-ray lines from the annihilation of weakly interacting massive particles; nuclear recoils in direct detection; and direct production of the weakly interacting massive particle pair in high-energy colliders. Considering recent evidence in particular for a 130 GeV line from the Galactic center, we discuss the detection prospects at upcoming experiments.