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Register a new userThe supersymmetric interpretation of the EGRET excess of diffuse Galactic gamma rays
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W. de Boer
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Recently it was shown that the excess of diffuse Galactic gamma rays above 1 GeV traces the Dark Matter halo, as proven by reconstructing the peculiar shape of the rotation curve of our Galaxy from the gamma ray excess. This can be interpreted as a Dark Matter annihilation signal. In this paper we investigate if this interpretation is consistent with Supersymmetry. It is found that the EGRET excess combined with all electroweak constraints is fully consistent with the minimal mSUGRA model for scalars in the TeV range and gauginos below 500 GeV.
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The public data from the EGRET space telescope on diffuse Galactic gamma rays in the energy range from 0.1 to 10 GeV are reanalyzed with the purpose of searching for signals of Dark Matter annihilation (DMA). The analysis confirms the previously observed excess for energies above 1 GeV in comparison with the expectations from conventional Galactic models. In addition, the excess was found to show all the key features of a signal from Dark Matter Annihilation (DMA): a) the excess is observable in all sky directions and has the same shape everywhere, thus pointing to a common source; b) the shape corresponds to the expected spectrum of the annihilation of non-relativistic massive particles into - among others - neutral $pi^0$ mesons, which decay into photons. From the energy spectrum of the excess we deduce a WIMP mass between 50 and 100 GeV, while from the intensity of the excess in all sky directions the shape of the halo could be reconstructed. The DM halo is consistent with an almost spherical isothermal profile with substructure in the Galactic plane in the form of toroidal rings at 4 and 14 kpc from the center. These rings lead to a peculiar shape of the rotation curve, in agreement with the data, which proves that the EGRET excess traces the Dark Matter.
Elsaesser and Mannheim fit a contribution of Dark Matter Annihilation (DMA) to the extragalactic contribution of the galactic diffuse gamma ray flux, as deduced from the EGRET data by Strong, Moskalenko and Reimer.They find a WIMP mass of 515{+110}{-75} GeV and quote a systematic error of 30%. However, they do not include large systematic uncertainties from the fact that the determination of the extragalactic flux (EGF) requires a model for the subtraction of the Galactic flux from the data.The data used were obtained with a model without Galactic DM, so one expects additional uncertainty in the region where DMA contributes. Including a Galactic DMA contribution reduces the significance and the WIMP mass. The latter then becomes compatible with the Galactic excess of diffuse gamma rays, which posseses all the properties of DMA with a much higher significance than the extragalactic excess.
We attempt to build a model that describes the {it Fermi} galactic gamma-ray excess (FGCE) within a UV-complete Supersymmetric framework; we find this to be highly non-trivial. At the very least a successful Supersymmetric explanation must have several important ingredients in order to fit the data and satisfy other theoretical and experimental constraints. Under the assumption that a {it single} annihilation mediator is responsible for both the observed relic density as well as the FGCE, we show that the requirements are not easily satisfied in many TeV-scale SUSY models, but can be met with some model building effort in the general NMSSM with $sim 10$ parameters beyond the MSSM. We find that the data selects a particular region of the parameter space with a mostly singlino lightest Supersymmetric particle and a relatively light CP-odd Higgs boson that acts as the mediator for dark matter annihilation. We study the predictions for various observables within this parameter space, and find that searches for this light CP-odd state at the LHC, as well as searches for the direct detection of dark matter, are likely to be quite challenging. It is possible that a signature could be observed in the flavor sector; however, indirect detection remains the best probe of this scenario.
The diffuse galactic EGRET gamma ray data show a clear excess for energies above 1 GeV in comparison with the expectations from conventional galactic models. The excess is seen with the same spectrum in all sky directions, as expected for Dark Matter (DM) annihilation. This hypothesis is investigated in detail. The energy spectrum of the excess is used to limit the WIMP mass to the 50-100 GeV range, while the sky maps are used to determine the halo structure, which is consistent with a triaxial isothermal halo with additional enhancement of Dark Matter in the disc. The latter is strongly correlated with the ring of stars around our galaxy at a distance of 14 kpc, thought to originate from the tidal disruption of a dwarf galaxy. It is shown that this ring of DM with a mass of $approx 2cdot 10^{11} M_odot$ causes the mysterious change of slope in the rotation curve at $R=1.1R_0$ and the large local surface density of the disc. The total mass of the halo is determined to be $3cdot 10^{12} M_odot$. A cuspy profile is definitely excluded to describe the gamma ray data. These signals of Dark Matter Annihilation are compatible with Supersymmetry for boost factors of 20 upwards and have a statistical significance of more than $10sigma$ in comparison with the conventional galactic model. The latter combined with all features mentioned above provides an intriguing hint that the EGRET excess is indeed a signal from Dark Matter Annihilation.
We demonstrate that young star clusters have a $gamma$-ray surface brightness comparable to that of the diffuse Galactic emission (DGE), and estimate that their sky coverage in the direction of the inner Galaxy exceeds unity. We therefore suggest that they comprise a significant fraction of the DGE.
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