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Experiment finds hints of WIMPs: three potential signals of dark matter detected.

Ultracold crystals designed to catch particles of dark matter deep underground have come up with three potential detections, physicists reported April 13.

The researchers do not have enough evidence to say they have discovered dark matter particles, but the finding qualifies as a rare clue in the frustrating quest to understand the universe's most elusive substance.

"We do not believe this result rises to the level of discovery," said Kevin McCarthy, the MIT physicist who made the announcement. "But it does call for further investigation."

Dark matter has confounded scientists since the 1930s. A galaxy's stars, gas and dust cannot account for all of its mass, so astronomers think that some sort of elusive matter that does not absorb or emit light must outweigh ordinary matter by more than 5 to 1. Because astronomers cannot see dark matter, its identity has been difficult to figure out.

Theoretical physicists have put forth some ideas for particles that might constitute dark matter, including one called a weakly interacting massive particle, or WIME The hope is that even though dark matter doesn't often interact with regular matter, WIMPs may do so occasionally.

The experiment that made the newly reported detections is designed to pick up the signal of a WIMP as Earth passes through the Milky Way's sea of dark matter. The Cryogenic Dark Matter Search consists of a network of silicon and germanium crystals cooled to near absolute zero. The detectors sit in the Soudan Underground Laboratory in Minnesota, a former iron mine more than 700 meters beneath the surface.

If WIMPs exist, one should very occasionally slam into the nucleus of a silicon or germanium atom, causing a release of energy and a detectable vibration in the crystal. The hundreds of meters of earth above the experiment prevent other particles, such as protons and neutrons, from reaching the crystals and triggering a false positive (SN: 8/28/10, p. 22).

McCarthy reported that between July 2007 and September 2008, two of the experiment's 11 silicon detectors picked up a total of three signals consistent with those expected from WIMP interactions. If the signals were caused by WIMPs, McCarthy estimates the dark matter particle would weigh about 10 times the mass of the proton, well below many theoretical estimates. The results also appear in a paper posted online April 15 at

While the crystals' underground setup provides plenty of shielding, some nonWIMP particles, such as electrons on the crystals' surface, can cloud the results. The CDMS researchers sayit's extremely unlikely that three events would show up from nonWIMP sources.

But the energy released by the potential WIMPs is at the very lower limit of the detectors' sensitivity, warns Richard Gaitskell, a physicist at Brown University, making erroneous detections more likely. He also has concerns that the two crystals that picked up the signal could be more susceptible to false positives than the rest.

CDMS physicist Enectali Figueroa Feliciano of MIT joins Gaitskell in remaining cautious about the new data. In 2009, CDMS reported that its germanium detectors had snagged two potential WIMPs, but further analysis revealed them to be surface electrons.

Figueroa-Feliciano says he would be more convinced if the detectors had picked up 10 or 12 signs of WIMPs rather than just three. Even then, a definitive detection would require multiple experiments worldwide to converge on the same characteristics for a dark matter particle.

Many other experiments around the world are on the case. One in Italy called DAMA, short for Dark Matter, has made bold claims of dark matter detection that have drawn skepticism from many scientists. Other experiments have claimed to find signals at masses similar to this latest CDMS calculation but have not definitively said they have observed WIMPs.

"I'm more excited than I should be, but I can't help it," says Katherine Freese, a theoretical astrophysicist at the University of Michigan in Ann Arbor. "Multiple experiments seeing something at the same mass is pretty exciting."

The difficulty is that each experiment uses a different detection technique and has its own protocol for distinguishing WIMPs from background noise, making it hard to compare results.

As for CDMS, the silicon detectors that found these signals are no longer collecting data. Researchers recently upgraded the Soudan facility with supersensitive germanium detectors. Over the next few years, these germanium detectors will move to a new, deeper underground home in Sudbury, Ontario, about 2 kilometers below the surface.

Caption: A researcher examines detectors in the Cryogenic Dark Matter Search, deep underground in Minnesota. Vibrations of crystals have captured three possible signs of long-sought dark matter.


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Title Annotation:Atom & Cosmos; weakly interacting massive particles
Author:Grant, Andrew
Publication:Science News
Geographic Code:1USA
Date:May 18, 2013
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