Second experiment hints at seasonal dark matter signal

Things just got a little less lonely for researchers who have been insisting for years not only that their experiment has found dark matter, but also that the dark matter signal varies with the seasons. Now a second experiment, called CoGeNT, is reporting similar findings, though both results are in conflict with two other teams' observations.

No one knows what dark matter is – astronomers merely detect its gravitational pull on normal matter, which it seems to outweigh by a factor of five to one. But many researchers believe it is made of theoretical particles called WIMPs, which interact only weakly with normal matter.

Since 1998, researchers running the DAMA experiment deep inside the Gran Sasso mountain in Italy have claimed to have found evidence of WIMPs.

DAMA uses an array of sodium iodide detectors to spot the rare moments when WIMPs slam into atoms in the detectors, producing flashes of light. The number of flashes ebbs and flows with the seasons, and DAMA team members argue that this is because Earth's velocity relative to the surrounding sea of dark matter changes as the planet orbits the sun. They say their observations could be explained by a WIMP weighing a few gigaelectronvolts.

Tense situation

However two other experiments have found no sign of dark matter with their detectors. One, called XENON100, uses 100 kilograms of liquid xenon deep below Gran Sasso mountain, and the other, called CDMS II, uses ultra-pure crystals of germanium and silicon housed in a deep mine in Soudan, Minnesota.

Both experiments are so sensitive that they should have seen dark matter if the DAMA result is due to WIMPs. "The situation has created tension," says Dan Hooper, a theorist at the University of Chicago in Illinois.

Now another dark matter experiment called CoGeNT has found a seasonal variation in its results, reports team leader Juan Collar, who presented an analysis of 442 days of observations at the American Physical Society meeting in Anaheim, California, on Monday.

"We tried like everyone else to shut down DAMA, but what happened was slightly different," Collar said during his presentation.

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