Has a Particle of Darkness Come to Light?

PARIS--A team of scientists working in a laboratory deep in Italy's Apennine Mountains has picked up the strongest hint so far of a strange new particle. Its appearance, which has yet to be confirmed, would not be entirely unexpected, but it would have profound implications. Reclusive and ponderous, with about 60 times the mass of the proton, such particles, called WIMPs (for weakly interacting massive particles), could account for some or all of the mysterious dark matter that astronomers believe far outweighs the galaxy's glowing stars and gas.

The collaboration, working at the Gran Sasso laboratory, set out to detect WIMPs with a contraption consisting of nine 9.7-kilogram crystals of doped sodium iodide--a material that generates a flash of light when one of its nuclei or electrons recoils after interacting with another particle. WIMPs should leave their signature in the form of a particle count that varies with the seasons, as Earth's orbit carries it into a galactic wind of WIMPs and then away again. The Gran Sasso data are "in favor of this modulation" with about a 99% level of statistical confidence, says Pierluigi Belli, a University of Rome physicist who is a member of the collaboration, called DAMA (for DArk MAtter). Belli announced the result here on 16 December at a gathering called--despite the location--the Texas Symposium on Relativistic Astrophysics.

If WIMPs are real, they might settle a long-standing problem. The Milky Way and other galaxies spin so quickly that the gravity of their ordinary, luminous matter is not enough to keep them from flying apart. Perhaps 90% of the galaxies' mass has to consist of some unseen matter to add the extra gravitational glue. WIMPs have become a favorite candidate for fleshing out galaxies to the required mass, in part because they are natural consequences of some speculative theories of particle physics known as supersymmetry.

In sharp exchanges after Belli's talk, however, experimenters took aim at everything from the DAMA group's statistical analysis techniques to the fact that data presented so far cover mainly the rising part of the modulation. "This is a work in progress," Belli shot back. The collaboration is now analyzing additional data.