How convincing is the evidence for dark matter annihilation at 130 GeV in the galactic center from the Fermi Satellite data?

I listened to Christoph Weniger present his results at SLAC today. See his paper is here: http://arxiv.org/abs/1204.2797 and also see a different analysis here: http://arxiv.org/abs/1205.1045. The data seems convincing to me! Is this result consistent with theoretical expectations for DM candidates? In particular is the reported estimate cross section for annihilation into photons consistent with estimated cross sections for the various WIMP dark matter candidate particles (like LSP dark matter candidates)? Are there any other reasonable astrophysical mechanisms that would produce this 130 GeV photon line?

The summary for the talk claims: Using 43 months of public gamma-ray data from the Fermi Large Area Telescope, we find in regions close to the galactic center at energies of 130 GeV a 4.6 sigma excess that is not inconsistent with a gamma-ray line from dark matter annihilation. When taking into account the look-elsewhere effect, the significance of the observed signature is 3.3 sigma. If interpreted in terms of dark matter particles annihilating into a photon pair, the observations imply a partial annihilation cross-section of about $10^{-27} cm^3 s^{-1}$ and a dark matter mass around 130 GeV.

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You kinda answer your own question. One observation at better than 3 sigma. OK, so that's good for a first report (and very exciting), but nothing is nailed down. – dmckee♦ Jul 20 '12 at 3:10

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I'm a little concerned that "How convincing is this breaking news" is a bit of a subjective discussion rather than a real question, but I'll let it go a bit to see if we get non-subjective answers and to allow input from interested users. – dmckee♦ Jul 20 '12 at 3:11

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@dmckee The one particular piece of concrete information I don't know is whether the reported cross section is reasonable when compared to proposed DM candidates. I was hoping someone here could nail down that one concrete fact. If this reported cross section is many orders of magnitude too large, I would question it's validity, whereas if it is a reasonable cross section then I would think this evidence is more convincing. By the way, this is not brand new information - it has been reported since April/May 2012. – FrankH Jul 20 '12 at 3:18

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Well, that would certainly be a nice piece of concrete, non-subjective, non-discussion. – dmckee♦ Jul 20 '12 at 3:20

2 Answers

Another very fresh paper presented at Dark Attack yesterday, one by Hektor et al.,

http://arxiv.org/abs/1207.4466

also claims that the signal is there – not only in the center of the Milky Way but also in other galactic clusters, at the same 130 GeV energy. This 3+ sigma evidence from clusters is arguably very independent. All these hints and several additional papers of the sort look very intriguing.

There are negative news, too. Fermi hasn't confirmed the "discovery status" of the line yet. Puzzles appear in detailed theoretical investigations, too. Cohen at al.

http://arxiv.org/abs/1207.0800

claim that they have excluded neutralino – the most widely believed identity of a WIMP – as the source because the neutralino would lead to additional traces in the data because of processes involving other Standard Model particles and these traces seem to be absent. The WIMP could be a different particle than the supersymmetric neutralino, of course.

Another paper also disfavors neutralino because it is claimed to require much higher cross sections than predicted by SUSY models:

http://arxiv.org/abs/1207.4434

But one must be careful and realize that the status of the "5 sigma discovery" here isn't analogous to the Higgs because in the case of the Higgs, the "canonical" null hypothesis without the Higgs is well-defined and well-tested. In this case, the 130-GeV-line-free hypothesis is much more murky. There may still exist astrophysical processes that tend to produce rather sharp peaks around 130 GeV even though there are no particle species of this mass. I think and hope it is unlikely but it hasn't really been excluded.

Everyone who studies these things in detail may want to look at the list (or contents) of all papers referring to Weniger's original observation – it's currently 33 papers:

http://inspirehep.net/search?ln=en&p=refersto%3Arecid%3A1110710

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I'm reluctant to say much about this right now, right here, but: the cross section is a reasonable one for a loop annihilation to $\gamma\gamma$. From the model-building point of view the thing to worry about is that many DM models that predict such a loop would also predict much more frequent tree-level annihilation processes that are ruled out by the lack of continuum gamma rays from the same spot. From the astrophysical point of view the puzzle is why the gamma rays seem to come not quite from the galactic center but from a couple hundred parsecs away. And then there may be some other puzzling features in the data, in terms of similar lines being seen when looking in places where you don't expect to see dark matter.

Things will become much clearer over the next few months, if not faster, at which point more questions here might deserve longer answers.

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