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Candidates for dark matter particles bite the dust

Most sensitive experiment yet determines that earlier findings were artifacts

WIMP DETECTOR  Photomultiplier tubes such as these should detect subtle flashes of light caused by dark matter particles striking xenon nuclei.

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The elusive substance that makes up more than a quarter of the universe is now even more of a mystery. A supersensitive search for dark matter has come up empty, researchers announced in an October 30 press conference, casting serious doubt on the findings of experiments that have claimed detections or hints of dark matter particles.

Dark matter is the ultimate tease: Scientists know it permeates the universe because of its gravitational influence on distant galaxies, but they can’t see it and don’t know what it’s made of. Theoretical physicists have proposed that dark matter could come in the form of weakly interacting massive particles, or WIMPs, and predicted how those particles might behave. Scientists around the world have built giant underground experiments, shielded by soil and rock from the bombardment of other particles, to try to detect WIMPs.

The Large Underground Xenon detector, or LUX, is located in a former gold mine 1.5 kilometers beneath Lead, S.D. The experiment consists of a phone-booth-sized titanium tank that holds liquid and gaseous xenon. WIMPs rarely interact with ordinary matter, but every now and then one should collide with the nucleus of a xenon atom, causing two distinct flashes of light. Particles such as neutrons and electrons can cause similar signals upon striking xenon, but LUX researchers say they can eliminate false positives with unprecedented accuracy.

LUX operated for three months this summer and recorded 84 million potential detections, LUX physicist Richard Gaitskell of Brown University said. Yet after a detailed analysis, the LUX team found no signals that were convincingly caused by WIMPs.

The finding (or lack thereof) has implications for evaluating the results of other experiments. For example, in April physicists with the Cryogenic Dark Matter Search in Soudan, Minn., announced the possible detection of three WIMPs, each with a strikingly low mass about 10 times that of a proton (SN: 5/18/13, p. 10). If that were the correct mass, Gaitskell said, LUX should have detected more than 1,500 WIMPs because it is 20 times as sensitive to such low-mass particles as any previous experiment.

The LUX results also clash with claimed detections from the CoGeNT Dark Matter Experiment, also in Soudan, Minn., and the DAMA/LIBRA experiment near L’Aquila, Italy. “We are just not consistent with observations that other dark matter experiments have made,” Gaitskell said, adding that physicists in other experiments probably were fooled by false positives. “We’re ruling out those low-mass WIMPs.” LUX will follow up its initial observations with almost a year of operation, providing increased sensitivity that could reveal WIMPs currently sneaking just below the threshold for detection.

Katherine Freese, a theoretical astrophysicist at the University of Michigan in Ann Arbor, isn’t quite ready to give up on previous findings, even though she calls LUX really well done. She says that WIMPs may interact differently with xenon than they do with silicon, germanium and sodium iodide, which make up the detectors in other experiments. “I’m going to do my damnedest to save” low-mass WIMPs, she says. “But to be honest, it’s not looking too good.”

Editor's Note: This story was updated November 4, 2013, to correct the amount of time LUX will operate and the chemical makeup of other experiments' detectors.

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