Sunday, March 20, 2005

A Signal of Dark Matter Annihilation?

This afternoon (Saturday for me here in California, even though the blog will register this as a Sunday post) I chaired the first session of talks on the connections between cosmology and the International Linear Collider. The talks were all excellent, but I particularly enjoyed the one delivered by Wim de Boer, from the University of Karlsruhe in Germany.

Wim was concerned with data from the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory. This experiment discovered an excess of diffuse gamma rays in all directions on the sky, which has puzzled physicists for a while now. Wim's claim is that this observation is well explained by the hypothesis that the gamma rays arise from the annihilations of dark matter particles with each other. Working with this hypothesis, Wim and collaborators were able to reconstruct the distribution of dark matter in the galaxy required to yield the observed gamma ray excess. This is a very tempting idea and, if verified, it would be a wonderful piece of evidence for dark matter.

There is more to this story though. The predicted distribution contains a ring of excess dark matter in the galaxy. What is fascinating is that two years ago the Sloan Digital Sky Survey (SDSS) announced the discovery of a corresponding ring of stars in the galaxy, thought to be left over from the collision of a smaller, dwarf galaxy with our galaxy billions of years ago. It is natural that there would be an associated ring of dark matter in the same location. Together these two pieces of data may therefore be our first tantalizing hint of dark matter annihilation.

(Wim went further, considering a particular dark matter candidate - the neutralino in minimal supergravity (mSUGRA) models. This part was more technical and model-specific, and while it was also fascinating, I won't go into the details here.)

The last talk of the day was a colloquium on collider-cosmology connections. A year ago, at the LCWS2004 meeting in Paris, I gave this talk. This year Jonathan Feng delivered it and did a truly masterful job, providing material at a variety of levels to match the varied levels of expertise in the audience. These connections are important to particle physics and to cosmology and Jonathan described them perfectly.

All in all a great day of physics.
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