Dark Matter Experiment, After First Three-Month Run, Finds Nothing

By Laura Dattaro
Published: October 31, 2013

Photomultiplier tubes capable of detecting as little as a single photon of light line the top and bottom of the LUX dark matter detector. They will record the position and intensity of collisions between dark matter particles and xenon nuclei. (Matt Kapust/Sanford Underground Research Facility)

After its first three-month run, the latest dark matter experiment has yet to see hints of the elusive substance.

The Large Underground Xenon (LUX) experiment is buried at the Sanford Underground Research Facility, almost 5,000 feet below ground in a former gold mine in South Dakota, to prevent false signals from cosmic rays from space. It’s a vat filled with almost a third of a ton of liquid xenon, designed to observe any potential interactions with dark matter particles.

No such interaction has ever been observed, despite decades of searching. Dark matter still exists in the realm of the theoretical — gravitational effects on large celestial objects like galaxies alert us to its presence. For example, galaxies have been seen spinning faster than the rate at which we expect them to spin given their mass. Like all other types of matter, dark matter should be made of particles; it’s the exact nature of the particles, however, that is still unknown.

The leading candidates are called WIMPs — weakly interacting massive particles. At LUX, physicists hope one of the particles will collide with a xenon atom, emitting signals they could use to learn about the particle.

Though LUX’s first run didn’t turn up anything, researchers say they are happy with the precision of the experiment, according to The New York Times. “Just because we don’t see anything in the first run doesn’t mean we won’t see anything in the second,” physicist Richard Gaitskell, a LUX spokesman, told The Times. “In 25 years of searching, this is the cleanest signal I’ve ever seen.”

The run may have also ruled out low-mass candidates found by another experiment, the Cryogenic Dark Matter Search (CDMS) in Minnesota, Ars Technica reports, which found hints of possible dark matter particles earlier this year. If dark matter existed in the low-mass form posited by the CDMS results, LUX should have found more than 1,600 collisions in its three-month run, according to a news release, instead of the big zero it actually did. (Juan Collar, a dark matter specialist at University of Chicago, told The Times that the LUX detector may not have been “adequately calibrated to detect” the low-mass particles, noting that the physicists at LUX had promised to perform the needed calibrations to “settle the issue.”)

“This is only the beginning for LUX,” physicist Dan McKinsey, co-spokesman for LUX, said in the release. “Now that we understand the instrument and its backgrounds, we will continue to take data, testing for more and more elusive candidates for dark matter.”

There’s always the chance that dark matter doesn’t interact with regular matter at all, but researchers are betting on the fact that it does, and building ever larger and more sensitive detectors. LUX will run until 2015; its successor, the LUX-ZEPLIN, or LZ, will be a thousand times more sensitive.

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