Friday, 24 July 2009

FERMI is seeing something?

It has become a tradition that release of new astrophysical data proceeds in the atmosphere of scandal, sex, and intrigues. Less than two weeks ago in this blog I was whining that the FERMI collaboration is guarding their secrets too effectively. Not any more. Not even guns and barbed wire fences could keep theorists off, once they have smelled real data.

Once again the story is related to the searches of indirect signals of dark matter in cosmic rays. In the previous episodes, the PAMELA satellite reported an excess of cosmic ray positrons between 10 and 100 GeV, and FERMI announced that the spectrum of electrons and positrons is harder (falls off more slowly with energy) than predicted by conventional cosmic ray propagation models. Although there exist plausible explanations in terms of mundane astrophysics, the excess positrons and electrons can also be understood as products of annihilation or decay of dark matter in our galaxy. If that is the case, there is one robust consequence. The electrons and positrons produced by dark matter throughout the galaxy should interact with the photons of the cosmic microwave background and starlight in the process known as the inverse Compton scattering, ICS in short. A high energy electron scattering off a photon transfers most of its energy to the photon. Thus, dark matter models explaining the PAMELA and FERMI results also predict an excess of gamma ray photons from the galactic center at energies 100 GeV and more. That's why the astroparticle community has been eagerly awaiting the release of FERMI measurements of gamma rays from the galactic center.

A week ago FERMI announced some new results at the TeV Particle Astrophysics conference held at SLAC. The new data included measurements of the gamma ray spectrum from the galactic center. The results from the one-by-one degree square around the galactic center, while providing new constraints on dark matter models, do not show any exciting features, see the upper plot. However, the data from a larger portion of the sky referred to as the inner galaxy do show an excess, or a hardening of the spectrum, starting at 100 GeV, see the plot on the left. The hardening occurs exactly where the dark matter models predict it! The FERMI collaboration did not want to post the latter result because it is still contaminated with poorly understood backgrounds. But somehow, mysteriously, the plot made it into the summary talk given by Persis Drell and the slides were posted at the conference page. These slides have now been removed; too late alas too late. Today there is a new paper on arXiv that interprets the new FERMI data in terms of the PAMELA/FERMI motivated models dark matter. The plot below reproduced from that paper shows the FERMI data together with expected backgrounds and predictions from dark matter models.

So is FERMI seeing dark matter? Most likely not. Members of the FERMI collaboration suspect that the feature in the gamma ray spectrum around 100 GeV is due to an unexpected background from other cosmic ray particles. Further analysis should clarify the situation. What is definitely true is that we're living in interesting times...

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