APS 2009: Fermi seeing dark matter's signal?
Peter Michelson, head of the Large Area Telescope team on Fermi, the gamma ray telescope formerly known as GLAST, gave the opening talk this morning. He went through all the amazing things that it has found in its first 8 months: gamma-ray only pulsars, milli-second pulsars, and active galactic nuclei. But he saved the news for last: Fermi, like two other experiments PAMELA and ATIC, is seeing way too many electrons and positrons all around us -- which could be an indirect signal from the annihilation or decay of dark matter, the stuff that makes up up to a quarter of the mass of the universe, but has yet to be detected directly as a particle.
Last year, the PAMELA and ATIC teams showed rises in positrons and electrons -- far more than are expected to be in the diffuse galactic background. Now, Fermi, shows not just a rise, but a bump --- centered around 300 or 400 gigaelectronvolts. That bump could mark the center of mass for a dark matter particle, such as a WIMP (Weakly Interacting Massive Particle). The results of the different experiments are not exactly the same (Fermi counts the total electrons and positrons, whereas PAMELA can distinguish between the two), but they seem to be compatible. Michelson isn't ready to rule out a conventional source just yet -- the extra particles could be generated from nearby pulsars, and then whipped into a diffuse background by galactic magnetic fields -- but he says that there is a good chance that they could be observing new physics. "Exciting stuff," he says. Symmetry Breaking has their take on the discovery here. More news to come -- after I learn some more myself.
[Editor's update: that full story is now available here.]
