The Large Underground Xenon (LUX) dark matter experiment, which operates beneath a mile of rock at the Sanford Underground Research Facility in the Black Hills of South Dakota, has completed its silent search without detecting the missing matter of the universe.
Dark matter is thought to account for more than four-fifths of the mass in the universe. Scientists are confident of its existence because the effects of its gravity can be seen in the rotation of galaxies and in the way light bends as it travels through the universe, but experiments have yet to make direct contact with a dark matter particle. The LUX experiment was designed to look for weakly interacting massive particles, or WIMPs, the leading theoretical candidate for a dark matter particle. If the WIMP idea is correct, billions of these particles pass through your hand every second, and also through the Earth and everything on it. But because WIMPs interact so weakly with ordinary matter, this ghostly traverse goes entirely unnoticed.
Although the LUX experiment failed to directly detect dark matter, the research results will help constrain models for what dark matter could be. The findings eliminated a large swath of mass ranges and interaction-coupling strengths where WIMPs might exist. And the meticulous work of LUX scientists will aid future direct detection experiments.
“LUX’s sensitivity far exceeded the goals for the project, but yielded no trace of a dark matter particle," said UC Davis physics professor Mani Tripathi, a project principal investigator. "LUX’s extreme sensitivity makes the team confident that if dark matter particles had interacted with the LUX’s xenon target, the detector would almost certainly have seen it.”
Tripathi is also involved in the CMS experiment at the Large Hadron Collider (LHC), which is searching for dark matter by producing it in proton-proton collisions. The world will have to wait and see if the new run this year at the LHC will show evidence of dark matter particles, or if the discovery occurs in the next generation of larger direct detectors.
The new results were presented today (July 21) by Aaron Manalaysay, the Analysis Working Group Coordinator of the LUX experiment and an assistant project scientist at UC Davis.
“These careful background-reduction techniques and precision calibrations and modeling, have enabled us to probe dark matter candidates that would produce signals of only a few events per century in a kilogram of xenon,” said Manalaysay, who described the results at an international dark matter conference in Sheffield, UK. Further details on the results may be found at the LUX Collaboration’s website.
The 20-month run of LUX represents one of the largest exposures ever collected by a dark matter experiment, the researchers said. The LUX detector consists of a third-of-a-ton of cooled liquid xenon surrounded by powerful sensors designed to detect the tiny flash of light and electrical charge emitted if a WIMP collides with a xenon atom within the tank. The detector’s location at Sanford Lab beneath a mile of rock, and inside a 72,000-gallon, high-purity water tank, helps shield it from cosmic rays and other radiation that would interfere with a dark matter signal.
— Adapted from a Berkeley Lab press release