It's a lazy summer season: everybody's on the beach and nothing's much going on. To stay in business I have to feed you with some microwaved news. Last week the FERMI collaboration uploaded a pile of papers on arXiv, one of which caught my attention. FERMI is a space gamma-ray observatory, but first of all he is a ruthless terminator with a mission to eliminate other astrophysical experiments. A while ago in May the widely publicized measurement of the cosmic-ray electron+positron spectrum pierced the ATIC balloon that had been pumped for several months. Earlier this year FERMI made another kill: it shot down EGRET, its direct predecessor in cosmic gamma-ray observations. That result has been presented at conferences for a few months, but only last week it made it to arXiv (there's also a longer PRL paper announced).
EGRET was a sattelite gamma-ray observatory that in particular studied the diffuse gamma ray emission in the 30 MeV-100 GeV range. Diffuse means spread over the sky rather than originating from point sources. The main source of diffuse radiation is scattering of the cosmic rays on the milk of the Milky Way. Dark matter annihilation into standard model particles can also contribute to the diffuse flux. The EGRET results showed an excess of gamma rays above 1 GeV, which was quickly hailed as the harbinger of dark matter and supersymmetry. But FERMI's measurement now demonstrates that there's nothing exciting going on below 10GeV: the experimental curve nicely follows the theoretical prediction of the standard propagation model. No exotic physics in sight. ATIC, EGRET...who's next?
FERMI's goal is to measure the diffuse gamma-ray spectrum up to some 300 GeV. The high energy data around are even more interesting for theorists as many popular models of dark matter - especially those that explain the PAMELA positron excess - predict a large signal peaking around a few hundred GeV. More results are expected in August since on August 12th the collaboration is supposed to make all their photon data public. If you hear cries and squealing later this summer that's the dark matter models being slaughtered. Or maybe FERMI sees an excess in which case all hell will break loose? The rumor is...the weird thing is that there are no rumors. Last time, quite accurate descriptions of the electron spectrum circulated in the community months before publication of the FERMI data, and theory papers from outside of the collaboration were out days after FERMI's publication (or even violating causality in one case). Probably because of that experience the collaboration has now entrenched in its camp with barbed wire fences, dogs, booby traps to keep off the theorists. Oh come on, dont be so serious, we also wanna know ;-)