World’s most sensitive dark matter detector finishes search

Despite searching for a period of 20 months, a device heralded as the world’s most sensitive dark matter detector was unable to find even a hint of the invisible particles believed to account for up to 80% of the mass in the universe, an international team of scientists has revealed.

Speaking Thursday at the Identification of Dark Matter Conference (IDM2016) in Sheffield, UK, researchers involved with the Large Underground Xenon (LUX) dark matter experiment in South Dakota said the equipment’s sensitivity had “far exceeded” the project’s goals and that had it interacted with any dark matter particles, LUX “almost certainly” would have detected them.

“LUX has delivered the world’s best search sensitivity since its first run in 2013,” Rick Gaitskell, a physics professor at Brown University and co-spokesperson for the LUX Collaboration, said in a statement. “It would have been marvelous if the improved sensitivity had also delivered a clear dark matter signal. However, what we have observed is consistent with background alone.”

On the positive side, Gaitskell noted that during the final 20-month run (which began in October 2014 and ended in May 2016), scientists involved in the collaboration were able to push the dark matter detector’s sensitivity to levels “four times better than the original project goals.”

No evidence of WIMPs detected during 20-month run

Based on the performance of the detector’s xenon target, the researchers were able to eliminate many potential dark matter particle models, which they said will help direct future experiments designed to hunt for the source of this hypothesized, mysterious, nonluminous material.

According to Gaitskell and his colleagues, dark matter is believed to account for roughly four-fifths of the mass in the observable universe, and while it has never been seen, scientists believe that it exists based on how it influences gravity in galaxies and in the way that light bends as it travels through the cosmos. To date, though, no experiment has detected a dark matter particle.

Located in the Sanford Underground Research Facility in the Black Hills of South Dakota, the LUX project was developed to search for one particular type of dark matter candidate – weakly interacting massive particles (WIMPs). Scientists theorized that these particles are everywhere, but since they interact weakly with normal matter, they go completely unnoticed.

Using a detector comprised of 600-plus pounds of cooled liquid xenon and a powerful array of sensors, LUX was designed to detect the minuscule electrical charge and light flash emitted when WIMPs collide with xenon atoms within the detector’s tank. Since the tank is underground, it is protected from external sources of radiation, preventing interference with dark matter signals. Despite analyzing nearly one-half million gigabytes of data during its 20-month run, however, LUX was unable to detect any evidence of WIMP collisions.

“We worked hard and stayed diligent over more than a year and a half to keep the detector running in optimal conditions and maximize useful data time,” Berkeley Lab physicist Simon Fiorucci, science coordination manager for the experiment, said. “The is unambiguous data we can be proud of and a timely result in this very competitive field – even if it is not the positive detection we were all hoping for.”

Findings will greatly benefit future dark matter detection projects

The results obtained by the LUX experiment eliminated a large number of different mass ranges and interaction-coupling strengths where WIMPs were thought to exist, but it has not eliminated the possibility that the weakly-interacting particles themselves are real, Gaitskell emphasized. In fact, the WIMP model “remains alive and viable,” he added.

While they were unable to pinpoint dark matter particles, the research team said that their work will be valuable in shaping future detection experiments. “LUX has done much more in terms of its sensitivity and reliability than we ever expected it to do,” Gaitskell noted. “We always want more time with our detectors, but it’s time to take the lessons learned from LUX and apply them to the future search for dark matter.”

Members of the LUX Collaboration, including scientists from 20 universities and laboratories in the US, the UK and Portugal, will spend the next few months analyzing the data collected by the experiment. They hope to find additional information that will benefit future dark-matter hunting projects, including LUX’s direct successor, the LUX-ZEPLIN (LZ) experiment.

“The global search for dark matter aims to answer fundamental questions about the makeup of our universe. We’re proud to support the LUX collaboration and congratulate them on reaching this higher level of sensitivity,” said Mike Headley, executive director of the South Dakota Science and Technology Authority (SDSTA), which manages both experiments. “We’re looking forward to hosting the LUX-ZEPLIN (LZ) experiment, which will provide another major step forward in sensitivity.”

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Image credit: C. H. Faham

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