Experiment detects particles of dark matter, maybe

Events in underground experiment too few for certainty, but match the signature of WIMPs

Analyzing results of an experiment in a northern Minnesota mine, physicists report the possible detection of particles of dark matter — the proposed invisible material believed to account for about 80 percent of the mass of the universe. The physicists caution, however, that there’s about a one in four chance that ordinary subatomic particles, rather than dark matter, could account for the signals.

WAITING FOR WIMPS Thirty germanium detectors (one shown here) are being used to search for WIMPs in the CDMS experiment, housed in the Soudan mine in northern Minnesota. Fermilab
A LOOK INSIDE An underground experiment has yielded possible signatures of dark matter particles called WIMPs. This view shows the inner layers of a cryostat with two of the dark matter experiment’s five towers of detectors installed. Detector towers are mounted in the holes covered by hexagonal plates. CDMS Collaboration, Fermilab

The experiment, called the Cryogenic Dark Matter Search, relies on 30 detectors made of germanium and silicon crystals cooled to just above absolute zero. The detectors record tiny vibrations imparted by a proposed type of dark matter called weakly interacting massive particles, or WIMPs. WIMPs streaming in from space would very rarely jostle the germanium nuclei, some 800 meters underground in the Soudan mine, generating a tiny amount of heat and slightly altering the charge on the detectors in a characteristic pattern.

In new analyses of data recorded in 2007 and 2008, researchers identified two events that might be attributed to WIMPs. Two members of the team, Jodi Cooley of Southern Methodist University in Dallas and Lauren Hsu of the Fermi National Accelerator Laboratory in Batavia, Ill., reported the findings December 17 during separate presentations. Cooley spoke at the SLAC National Accelerator Laboratory in Menlo Park, Calif., while Hsu spoke at Fermilab.

The detection is far from definitive, because radioactive decay of ordinary material in the mine could be responsible for about 0.8 events, on average, during the same time period, the CDMS physicists calculate. Because of statistical fluctuations in the estimated rate of background events, there is a 23 percent chance that ordinary particles could have been responsible for both events attributed to WIMPs, Hsu says. Physicists typically require a much lower chance that a signal is spurious before regarding a result as conclusive.

“The result of this analysis cannot be interpreted as evidence for WIMP interactions but we also cannot reject on an individual basis either of these events as [a real dark matter] signal,” Hsu said during her talk.

But even now, the findings are “potentially very exciting,” says theorist Craig Hogan of the University of Chicago and Fermilab, who is not on the CDMS team. He adds that he is impressed with how precisely the CDMS researchers have calibrated the expected background of ordinary particles in the experiment.

Three or four more WIMP-like interactions recorded over the next few years by the experiment, now being upgraded with detectors containing three times as much germanium, would constitute proof of dark matter, Hogan says.

“That would be a huge transformation in how we do science,” he notes. “We would have a new form of matter to study.”

The late astronomer Fritz Zwicky first proposed the existence of dark matter in the 1930s when he calculated that the amount of ordinary matter in the Coma cluster of galaxies wasn’t enough to keep the cluster from flying apart. Additional, unseen material could provide the extra gravitational tug, he suggested. Since the 1970s astronomers have accumulated further evidence that the Milky Way and other galaxies are bathed in dark matter.

“We’ve seen evidence from many parts of the universe that dark matter is out there,” Hsu said.

Depending on the exact nature of dark matter, it could unify the subatomic world with the cosmic canvas. While astronomers need dark matter to explain the growth and motions of galaxies, particle physicists who subscribe to a theory called supersymmetry have proposed that every subatomic particle has an as yet undetected heavier partner. The least massive, electrically neutral of those partners might be the WIMP.

It was only a month ago that the researchers realized that they had found possible evidence of dark matter. During a videoconference on November 5, several members of the team presented independent findings on data analyzed in 2007 and 2008. To avoid unintentional bias, those analyses cataloged all the signals without focusing on those that might match WIMP signatures. Then, as the videoconference continued, Zeeshan Ahmed of Caltech and Matthew Fritts of the University of Minnesota in Twin Cities ran their own analyses. Ahmed’s results popped up first on a computer screen, revealing the detection of two candidate WIMPs. For the next 30 seconds, there was stunned silence on the videoconference, broken by a cacophony of excited comments.

WIMP fingerprints might be detected by a slew of experiments on the ground and in space (SN: 5/10/08, p. 12, p. 8; SN: 9/27/08) including collisions of high-energy protons at the world’s most powerful atom smasher, the Large Hadron Collider near Geneva. Physicists using the LHC, which has now resumed operation after a year of repairs, plan to look for what would be missing: a deficit of energy in the collision debris could be evidence for dark matter particles. (SN: 7/19/08, p. 16).

As other technologies to look for dark matter continue to blossom, “I think we’ll see more such announcements” of possible dark matter detections in the next few years, says Hogan

Other dark matter experiments, such as the Chicagoland Observatory for Underground Particle Physics, which uses bubble chambers to search for the interaction between dark matter and ordinary matter, might yet scoop CDMS in finding compelling evidence for the dark stuff, Hogan says.

In addition, several orbiting telescopes, including the Fermi Gamma-ray Space Telescope (SN Online: 5/2/09), are indirectly searching for dark matter by examining regions, such as the center of the Milky Way, where the invisible material is suspected to be densest. An excess number of gamma-rays or of pairs of particles and antiparticles in such regions might be produced when two WIMPs collide and annihilate each other.

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