Abstract
The analysis of the gamma-ray photonscollected by the Fermi Large Area Telescope reveals, after removal of astrophysical background,the existence of an excess towards the Galactic center.This excess peaks around few GeV, and its origin is compatible with the gamma-ray flux originating from Dark Matter annihilation.In this work we take a closer look on this interpretation; we investigate which kind of Dark Matter, and which type of interactionswith the Standard Model fields are able to reproduce the observed signal. The structure of the paper is twofold.In the first part, we follow an effective field theory approach considering both fermionic and scalar Dark Matter.The computation of the relic density, the constraint imposed from the null result of direct searches, and the reliabilityof the effective field theory description allow us to single out only two viable dim-6 operators in the case of fermionic Dark Matter.In the second part, we analyze some concrete models. In particular, we find that the scalar Higgs portal can provide a simple,concrete and realistic scenario able to explain the GeV excess under scrutiny.
Highlights
The quest for the first non-gravitational evidence of Dark Matter (DM) in the Galaxy is becoming, as time goes by, one of the most intriguing and challenging tasks in astrophysics.Paradoxically though it may seem – DM should be dark, after all – one of the most promising signals that could reveal the elusive fingerprints of DM is the analysis of the gammaray photons collected by the Fermi Large Area Telescope (LAT)
A number of recent analysis, in particular, point towards the possible existence of a residual excess originating from the Galactic Center (GC), peaked at few GeV, and compatible with the annihilation of DM particles with a thermally averaged cross section, σv ∼ 10−26 cm3s−1 [1,2,3,4,5], that is of the same order as the one required if DM is in thermal equilibrium in the early Universe
As already mentioned in the Introduction, the analysis performed in Refs. [6, 7] reveals that the energy spectrum of the Fermi bubbles arises from the combination of two different components: i ) an Inverse Compton Scattering (ICS) component, dominant at high latitudes, produced by a population of high-energy electrons trapped inside the bubbles, and ii ) an additional component, responsible for a bump at Eγ ∼ 1 − 4 GeV, compatible with DM annihilation, and dominant at low latitudes
Summary
The quest for the first non-gravitational evidence of Dark Matter (DM) in the Galaxy is becoming, as time goes by, one of the most intriguing and challenging tasks in astrophysics. A common explanation relies on the assumption of the existence of a spatially extended population of high-energy electrons trapped inside the bubbles, emitting gamma rays via Inverse Compton Scattering (ICS) on the ambient light This hypothesis allows to establish, via synchrotron radiation in the presence of microgauss magnetic field, an interesting spatial correlation with the WMAP haze [8] observed in the microwaves. Starting from the assumption that DM is a Weakly Interacting Massive Particle (WIMP), the best-suited tool to perform this analysis is the framework provided by the effective field theories This approach, integrating out the details of DM interactions at small distances, has the benefit of capturing a model-independent picture of the scrutinized signal, reaching general conclusions that can be used as guidelines for more complicated and concrete models. We collect all the relevant analytical formulas in Appendix B, while we quickly review the integration of the Boltzmann equation in Appendix C
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