By Ron Cowen
An experiment in Minnesota is the first to bolster a long-contested claim that detectors a continent away have found evidence of particles called WIMPs.
WIMPs are theorized particles considered to be leading candidates for dark matter, invisible material believed to make up more than 80 percent of the matter in the universe. In the Minnesota experiment, called COGENT, a hockey puck–sized chunk of germanium deep in a former iron mine attempts to record rare collisions with WIMPS.
In 15 months’ worth of data, COGENT researchers detected a seasonal variation in the collision rate — higher in summer and lower in winter — similar to that seen for 13 years by a larger experiment, using different detectors, in Italy. Researchers with that experiment, DAMA/LIBRA, have attributed the results to the Earth’s motion through a cloud of WIMPs (for weakly interacting massive particles) (SN: 5/10/08, p. 12). But many physicists have doubted that interpretation because, until now, no other experiment had found similar results.
COGENT team leader Juan Collar of the University of Chicago presented the new findings May 2 in Anaheim, Calif., at a meeting of the American Physical Society. Collar said he would not discuss the results with reporters until after papers describing the work are posted online.
The new COGENT results have features that you would expect from a dark matter detection, “something pretty similar to what DAMA has seen,” says theorist Dan Hooper of the Fermi National Accelerator Laboratory in Batavia, Ill., who heard Collar’s talk. “Everything that you would hope would be there, if it’s dark matter, is basically there.”
The finding does not constitute a discovery of dark matter, however, because the likelihood of such results appearing by chance are too high to qualify for what physicists consider proof, notes Hooper.
WIMPs are favored candidates for dark matter particles because they have been predicted in theories attempting to unify nature’s known particles and forces. Features predicted for WIMPs correspond with properties expected for cosmic dark matter, the gravitational glue that holds galaxies together and allowed them to form in the first place (SN: 8/28/10, p. 22).
If correct, the COGENT result implies that WIMPs have a relatively low mass, roughly five to 10 times the mass of a proton, says Katherine Freese of the University of Michigan in Ann Arbor. Recent results from other experiments appear to be in conflict with the experiment but may not be as sensitive as COGENT is to collisions with such low-mass particles (SN: 5/7/11, p. 12).
The COGENT finding intrigues Freese, who, with other researchers, predicted in the 1980s that a seasonal variation could be a sign of dark matter. That idea envisions vast halos of WIMPs engulfing galaxies, including the Milky Way. Earth would plow through the Milky Way’s WIMP halo faster in summer than winter because its revolution around the sun would then correspond with the rotation of the galaxy into the WIMP halo. In winter, the Earth’s revolution would take it in the opposite direction of the galaxy’s rotation. So detectors ought to see more collisions in summer than winter — the trend now seen by both DAMA/LIBRA and COGENT.
Theorist Neal Weiner of New York University says his calculations indicate that the COGENT team was either very lucky or saw an unusually high variation in the collision rate to be able to make a tentative detection with just 15 months of data.
Theorists are now increasingly challenged to identify properties of dark matter that might explain why some experiments see signs of the particles while others do not, Weiner said May 2 in Baltimore at a meeting on dark matter.
