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

Abstract

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 or MiDM), 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 WIMPs 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 formχχFµν F µν . 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 WIMPs; nuclear recoils in direct detection; and direct production of the WIMP 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.

Compressing the inert doublet model

“…For very small neutral mass splittings the A → HZ * decay can be displaced, and searches for a mono-jet with a soft displaced vertex are also possible, as suggested for models of inelastic DM in Refs. [105,106,107]. In the IDM, we find that macroscopic decay lengths cτ > 1 mm only occur for ∆ 0 1 GeV, and thus the displaced decay products of the A → HZ * decays are typically too soft to be identified by the LHC detectors.…”

Direct detection of dark matter polarizability

“…Later, the authors of ref. [7] emphasized that DM scattering for arbitrary masses proceeds via a photon loop, and estimated the rate using an effective field theory for the nucleus. More recent work on the DD signatures of (1.1) can be found in [8] and [9].…”

“…There are two types of direct detection signatures that (1.1) can lead to: an elastic scattering XT → XT (here T stands for the target nuclei) or an inelastic process XT → XT γ [7]. The first is numerically a loop effect.…”

Enhanced diphoton rates at Fermi and the LHC

“…Radio telescopes might help to confirm or rule out the dark matter interpretation of the line soon [32]. While the origin of the γ-ray line from the galactic center still has to be clarified, a noticeable amount of theoretical interest has been triggered [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]. If interpreted in terms of dark matter, the γ-ray line requires a rather large annihilation cross section into photons, σv γγ ∼ 10 −27 cm 3 s −1 , if one assumes an Einasto profile [22].…”