Fermilab

More hints of dark matter have emerged from the Cryogenic Dark Matter Search (CDMS), which hunts for the theorized particles from the depths of a mine in Minnesota.

Eight silicon detectors recorded three events that may represent collisions from weakly interacting massive particles, or WIMPs. Physicists have found hints of the existence of WIMPs before, but they remain elusive. Two other possible detections from the CDMS search, reported in 2010, turned out to be indistinguishable from background collisions from other, non-WIMP, sources. The same may yet hold true for the latest findings.

The work was reported on 13 April at the American Physical Society meeting in Denver, Colorado, and is published at arXiv.org. “We do not believe this result rises to the level of a discovery, but it does call for further investigation,” said Kevin McCarthy, a CDMS team member from the Massachusetts Institute of Technology in Cambridge.

CDMS-II, the second generation of the search, ran between 2003 and 2008. The earlier WIMP suspects were spotted in its 19 germanium detectors. The new work comes from a subset of its 11 silicon detectors, which are more sensitive than germanium to collisions from low-energy particles.

Cooled to a temperature of just 40 millikelvins, the CDMS-II detectors sense heat given off when a particle collides with one of their crystals. The challenge is distinguishing a possible WIMP collision from the many collisions of other particles, such as neutrons.

The CDMS tries to get around that by shielding its detectors as much as possible and by precisely calculating the rate of expected collisions from other, background sources. The three possible WIMP events popped out of data in which 0.7 similar events would be expected from background, McCarthy said. Two of them occurred in the same detector.

He reported the signal at a 99.81% confidence level, or around three sigma in statistical language. “We favor the WIMP plus background hypothesis,” he said.

But the CDMS-II result would imply a WIMP with a mass of 8.6 gigaelectronvolts, far lighter than physicists might expect. The experiment’s successor, SuperCDMS, is now running in the Soudan mine and may yet yield more insight.

Other dark-matter experiments are searching for WIMPs with tanks of liquid xenon. These include LUX in South Dakota’s Homestake mine and XENON1T in Gran Sasso, Italy. They may soon help to narrow down the possibilities for WIMP dark matter, says Tom Shutt of Case Western Reserve University in Cleveland, Ohio.