No WIMPs Here: Dark Matter Search Draws a Blank

When it comes to hunting WIMPS, an elusive but perhaps not hypothetical dark matter particle, not finding one at least sheds lights on where to look next.

Image: The Large Underground Xenon (LUX) dark matter detector is seen here after it was installed at the center of a water tank later filled with more than 70,000 gallons of ultra-pure deionized water, part of an experiment to look for dark matter particles. Credit: Sanford Underground Research Facility Scientists on Thursday said they were finished with a 20-month search for WIMPS, or weakly interacting massive particles, which took place inside an abandoned gold mine a mile beneath solid rock in the Black Hills of South Dakota.

Their WIMP detector consisted of a tank of cooled liquid xenon that was surrounded by sensors to measure tiny flashes of light caused by dark matter particles colliding with xenon atoms.

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To shield against cosmic rays, the tank of xenon was inside a 72,000-gallon tank of ultra-pure water.

Scientists modeled what a WIMP crashing into a xenon atom would look like and honed their hunting skills by blasting neutrons and radioactive gases into the detector.

Though the Large Underground Xenon, or LUX, experiment was four times more sensitive than original expectations, it was unable to detect any dark matter particles, scientists said Thursday at a conference in Sheffield, United Kingdom.

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While the experiment eliminates some WIMP candidates, it opens the door to other particle models that will need a larger and more sensitive experiment to detect, said Brown University physicist Richard Gaitskell.

A dark matter particle remains the leading theory to explain the universe's so-called "missing mass." Roughly 80 percent of the universe's mass cannot be directly detected. Its existence is inferred by measuring its gravitational impacts on the motions of galaxies and on the bending of light passing by.

For now, the hunt for WIMPS moves to the Large Hadron Collider (LHC) at CERN in Geneva, the world's most-powerful atom smasher.

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"LHC is looking to observe new particles that would provide the first evidence of a new theory that goes beyond the existing Standard Model. It is likely that if they see new particles, the associated physics will also provide an explanation for how dark matter particles were produced in the early universe," Gaitskell wrote in an email to DNews.

"At present, LHC has not seen any clear evidence for such new physics. We are waiting to see their next new analyses in the coming weeks," he added.

Evidence of for dark matter also could come from the Alpha Magnetic Spectrometer, or AMS, a particle detector attached to the International Space Station.

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AMS looks for charged particles, such as protons and anti-protons, which could stem from the annihilation of dark matter particles in the galaxy.

Scientists also are planning a LUX follow-on experiment that will be 70 times more sensitive than the original.

Construction of LUX-ZEPLIN (LZ), which will use 10 tons of liquid xenon compared to LUX's one-third ton, is underway. The detector is expected to begin operations in 2020, said physicist Harry Nelson, with the University of California, Santa Barbara.