By Andrew Grant
DENVER — Ultracold crystals designed to catch particles of dark matter deep underground have come up with three potential detections, physicists reported April 13 at a meeting of the American Physical Society.
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 actually see dark matter, they have no idea what it’s made of.
Theoretical physicists have put forth some ideas for particles that might constitute dark matter, including one called a weakly interacting massive particle, or WIMP. 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 galaxy’s sea of dark matter. The Cryogenic Dark Matter Search consists of a network of silicon and germanium crystals cooled to near absolute zero. It sits 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.
McCarthy reported that between July 2007 and September 2008, two of the experiment’s 11 silicon crystal detectors picked up three signals consistent with those expected from WIMP interactions. If the signals were caused by WIMPs, McCarthy estimates the dark matter particle would weigh in at 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 arXiv.org.
While the crystals’ underground setup provides plenty of shielding, some non-WIMP particles, such as electrons on the crystals’ surface, can cloud the results. The CDMS researchers say it’s extremely unlikely that three events would show up from non-WIMP 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 WIMP 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 published a paper reporting that its germanium detectors had snagged two potential WIMPs, but further analysis revealed them to be surface electrons. “This is a very tough business,” he says. “You can’t start jumping up and down.”
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 who happens to be attending a workshop in Ann Arbor on low-mass dark matter particles. “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.
“These are contentious times,” McCarthy says. “Everyone is pushing their detectors to the limit.”
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, the germanium detectors will move to a new, deeper underground home in Sudbury, Ontario, about 2 kilometers below the surface.
