Turning off the lights: How dark is dark matter?

Abstract

We consider current observational constraints on the electromagnetic charge of dark matter. The velocity dependence of the scattering cross-section through the photon gives rise to qualitatively different constraints than standard dark matter scattering through massive force carriers. In particular, recombination epoch observations of dark matter density perturbations require that ǫ, the ratio of the dark matter to electronic charge, is less than 10 −6 for m X = 1 GeV, rising to ǫ < 10 −4 for m X = 10 TeV. Though naively one would expect that dark matter carrying a charge well below this constraint could still give rise to large scattering in current direct detection experiments, we show that charged dark matter particles that could be detected with upcoming experiments are expected to be evacuated from the Galactic disk by the Galactic magnetic fields and supernova shock waves, and hence will not give rise to a signal. Thus dark matter with a small charge is likely not a source of a signal in current or upcoming dark matter direct detection experiments.

Can a millicharged dark matter particle emit an observable γ-ray line?

“…We will see that this bound already excludes an observable γ-ray line for these cases (hence a fortiori when the direct detection bounds are stronger, this is even more excluded). Note also that all these direct detection constraints assume a standard local DM density, which might not apply depending on how large the DM particle millicharge is, because it can be shielded by galactic magnetic fields [22,25]. For instance, when Q em > 10 −10 · (m DM /100 GeV), the depletion of the local DM relic density from magnetic shielding begins to be sizeable, therefore weakening the direct detection bounds.…”

“…An additional DM-baryon interaction such as the one provided by a millicharge modifies this picture by rendering DM effectively "baryonic". This affects the CMB power spectrum as well as the baryon acoustic oscillations, leading to the upper bound [20][21][22][23] σ…”

Composite millicharged dark matter

“…Millicharged dark matter can exchange photons with the nuclei and therefore constraints on ǫ can arise. Such constraints have been studied in the context of mirror dark matter [50] and in general [51]. In principle such a constraint might be quite severe.…”

“…Of course sufficiently small values of ǫ lead to little scattering and thus particles will avoid detection. Based on the CDMS data, the analysis of [51] provided an upper bound ǫ ≃ 10 −8 for millicharged dark matter with mass above 10 GeV, provided that millicharged particles are not evacuated from the galactic disc. This limit is of course evaded by our decaying dark pion with an ǫ ∼ 10 −12 , but it could exclude the thermally produced dark pion that requires ǫ ∼ 10 −5 .…”

Gamma rays from top-mediated dark matter annihilations

“…Such features occur when dark matter annihilates into a two-particle final state, with one of the particles a photon. Since dark matter must be electrically neutral (to good approximation [3]), such a signal is typically expected to be rather smaller than the expected annihilation cross section of a thermal relic, σv ∼ 3 × 10 −26 cm 2 /s (with mild dependence on the dark matter mass [4]). Nonetheless, such a gamma ray line is a very distinctive signal, unlikely to be faked by astrophysical backgrounds.…”