The empirical case for 10-GeV dark matter

Abstract

In this article, I summarize and discuss the body of evidence which has accumulated in favor of dark matter in the form of approximately 10 GeV particles. This evidence includes the spectrum and angular distribution of gamma rays from the Galactic Center, the synchrotron emission from the Milky Way's radio filaments, the diffuse synchrotron emission from the Inner Galaxy (the "WMAP Haze") and low-energy signals from the direct detection experiments DAMA/LIBRA, CoGeNT and CRESST-II. This collection of observations can be explained by a relatively light dark matter particle with an annihilation cross section consistent with that predicted for a simple thermal relic (σv ∼ 10 −26 cm 3 /s) and with a distribution in the halo of the Milky Way consistent with that predicted from simulations. Astrophysical explanations for the gamma ray and synchrotron signals, in contrast, have not been successful in accommodating these observations. Similarly, the phase of the annual modulation observed by DAMA/LIBRA (and now supported by CoGeNT) is inconsistent with all known or postulated modulating backgrounds, but are in good agreement with expectations for dark matter scattering. This scenario is consistent with all existing indirect and collider constraints, as well as the constraints placed by CDMS. Consistency with xenon-based experiments can be achieved if the response of liquid xenon to very low-energy nuclear recoils is somewhat suppressed relative to previous evaluations, or if the dark matter possesses different couplings to protons and neutrons.

Bremsstrahlung gamma rays from light dark matter

“…(5); 2) we increase the accuracy of the halo functions in the region of the Galactic Center (r 0.5 kpc) in order to be able to follow in detail the emission there; 3) we implement a refined map of the Interstellar Radiation Field (ISRF) 5 to compute ICS losses (and later the ICS emission). 6 The results that we obtain are crosschecked with Galprop under the same conditions, finding a reasonably good agreement: typically, the difference goes from the percent level to several tens of percent for extreme conditions; in the latter case, however, these are within the larger uncertainties that we wil discuss later. In general, what we aim at is a fully self-consistent calculation which correctly reproduces the main features of the results and, especially, the relative differences rather than the absolute values.…”

“…Namely, we shall concentrate on light WIMPs (O(10) GeV), which are motivated by several hints of putative signals in underground, direct detection experiments [3]-most recently by the 3σ level excess reported by Cdms-II [4]-as well as some gamma-ray and radio excesses [5], see also [6] for a review. We shall argue that, in presence of significant branching ratios into leptons, the role of the bremsstrahlung process in the e.m. field of the atoms of the interstellar gas alone can significantly alter the steady state electron/positron population as well as the resulting overall gamma emission in the (sub-)GeV range 1 .…”

“…For example, any quantitative interpretation of 'residual' gamma-ray as well as radio emission (in particular if in direction close to the Galactic Center or plane, as discussed in [5] and [6]) cannot prescind from accounting for the bremsstrahlung contribution: For a given model of astrophysical foreground injection, uncertainties in the gas distribution as well as technical approximations in the solutions of the relevant equations might alter the estimate of these residuals (see [7] for a discussion of related aspects). Even if significant 'excesses' are confirmed, the bremsstrahlung contribution can change the interpretation of the results (for example altering the best fit annihilation cross section, mass, and preferred final states).…”

Composite scalar dark matter

“…The spin-independent cross section for the elastic scattering of the DM candidate η off nuclei. The green shaded region is excluded by XENON100 [26], while the orange shaded region roughly corresponds to the excess events reported by DAMA, CoGeNT and CRESST-II [32,33]. The predictions of our scenario are labeled in the same way as in figure 1.…”

“…in ref. [32,33] shows that a DM candidate with m DM (10 − 20) GeV and σ SI (1 − 3) · 10 −41 cm 2 can account for the excess events reported by each of these experiments. This favoured region is shaded in orange in figure 4.…”