Wednesday, 9 May 2012

A dark matter signal from Fermi-LAT?

The Fermi gamma-ray telescope is a space-based gamma ray observatory. Somewhat unusually, for those of us more used to discussing collider experiments, the data it collects is freely available for anybody to analyse. Last month, Christoph Weniger, a dark matter theorist from the Max Planck Institute in Munich, released a paper in which he claimed to have discovered a peak in the gamma-ray flux, possibly corresponding to the annihilation of pairs of dark matter particles with a mass around 130 GeV. (I'm not sure, now, why it couldn't be from decay of a 260 GeV particle, and a quick skim over the paper hasn't enlightened me.) One interesting aspect of this research was that the data in which Weniger found a signal had already been analysed by the experimentalists themselves, and they concluded that there was no signal. Weniger's trick was to focus on those regions of the sky where models of the galaxy and its dark matter halo predict the signal-to-noise ratio to be high. Using the full dataset washes out the signal, hiding it in a (relatively) larger background.

This was actually discussed at length at our journal club at Oxford last week, and the general conclusion seemed to be that the work was very interesting, but far from convincing. It was suggested, for example, that the 'bump' could actually be the result of a background following a 'broken power law', where the flux falls with a particular power of the energy up to around 130 GeV, and then transitions to falling with a higher power above that. All in all, it seemed prudent to wait for independent evidence, or more data, before getting too excited.

We did not have to wait long! This week, another preprint came out which confirmed, using independent methods, the signal found by Weniger. The authors actually find an even sharper peak in the data, which makes it more difficult to explain away. They also claim that, due to the energy lost by high-energy photons through pair production of light fermions, the signal at 130 GeV should correspond to a dark matter mass of about 145 GeV. Furthermore, for the dark matter self-annihilation rate to be high enough to explain the data, the annihilation should be enhanced beyond the typical one-loop rate. This could occur if the dark matter annihilates via an s-channel resonance, that is, a particle with mass approximately 290 GeV (i.e. twice the dark matter mass), which couples directly to standard model fermions, and should be discoverable at the LHC.

I'm basically a spectator in the dark matter game, but it goes without saying that the discovery of particle dark matter would be very exciting, so this is one to watch with interest. I also expect that there will shortly be a flurry of mostly uninteresting papers 'predicting' a dark matter particle with a mass between about 125 and 150 GeV…

Edit: See also Luboš, who got to this in a more timely manner than I did.


  1. Has there been much discussion at the Oxford journal club of arXiv:1204.3924? There should be a few people who would be in a position to comment, and I imagine it is relevant to all the direct detection experiments.

    1. I don't think so, but it seems like a dramatic claim. I'll bring it up this week, and see what people think.

    2. There have been quite a few dramatic claims regarding dark matter recently. Sean Carroll pointed to as well a few days ago.

    3. It looks like they can't both be right! We seem to be closing in on dark matter, but it's proving itself to be quite elusive.

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