World's most sensitive dark matter detector completes search

Today at an international dark matter conference (IDM 2016) in Sheffield, UK, LUX scientific collaborators presented the results from the detector's final 20-month run from October 2014 to May 2016. The new research result is also described with further details on the LUX Collaboration's website. http://luxdarkmatter.org

LUX's sensitivity far exceeded the original expectations of the experiment, collaboration scientists said, but yielded no trace of a dark matter particle. 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 them. These new limits on dark matter detection will allow scientists to eliminate many potential models for dark matter particles, offering critical guidance for the next generation of dark matter experiments.

"LUX has delivered the world's best search sensitivity since its first run in 2013," said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment. "With this final result from the 2014-2016 run, the scientists of the LUX Collaboration have pushed the sensitivity of the instrument to a final performance level that is 4 times better than originally expected. 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."

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.

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.

The 20-month run of LUX represents one of the largest exposures ever collected by a dark matter experiment, the researchers said. The rapid analysis of nearly a half-million gigabytes of data was made possible with the use Brown University's Center for Computation and Visualization (CCV) and the advanced computer simulations at Lawrence Berkeley National Laboratory's (Berkeley Lab) National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy (DOE) Office of Science User Facility. Berkeley Lab is also the lead DOE laboratory for LUX operations.

"I am particularly pleased with the support LUX received from NERSC in processing these data," said Kevin Lesko, group leader of Berkeley Lab's Dark Matter group. "The Berkeley students, post-docs and visitors working on this analysis made extensive use of the NERSC for event scanning, calibration, Monte Carlo simulations and the data-blinding scheme."

Careful calibration

The exquisite sensitivity achieved by the LUX experiment came thanks to a series of pioneering calibration measures aimed at helping scientists tell the difference between a dark matter signal and events created by residual background radiation that even the elaborate construction of the experiment cannot completely block out.

"As the charge and light signal response of the LUX experiment varied slightly over the dark matter search period, our calibrations allowed us to consistently reject radioactive backgrounds, maintain a well-defined dark matter signature for which to search and compensate for a small static charge buildup on the Teflon inner detector walls," said Dan McKinsey, professor of physics at the University of California, Berkeley, senior faculty scientist at Berkeley Lab, and co-spokesperson for the LUX experiment.

"We worked hard and stayed vigilant over more than a year and a half to keep the detector running in optimal conditions and maximize useful data time," said Simon Fiorucci, a physicist at Berkeley Lab and Science Coordination Manager for the experiment. "The result 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."

The quest continues

While the LUX experiment successfully eliminated a large swath of mass ranges and interaction-coupling strengths where WIMPs might exist, the WIMP model itself, "remains alive and viable," said Gaitskell, the Brown University physicist. And the meticulous work of LUX scientists will aid future direct detection experiments.

Among those next generation experiments will be the LUX-ZEPLIN (LZ) experiment, which will replace LUX at the Sanford Underground Research Facility.

Compared to LUX's one-third-ton of liquid xenon, LZ will have a 10-ton liquid xenon target, which will fit inside the same 72,000-gallon tank of pure water used by LUX to help fend off external radiation. LZ is expected to have 70 times the sensitivity of LUX and will continue the search in 2020. "We're looking forward to hosting the LUX-ZEPLIN experiment, which will provide another major step forward in sensitivity," said Mike Headley, Executive Director of the South Dakota Science and Technology Authority (SDSTA).

LUX, the first major astrophysics experiment in the Davis Campus of the Sanford Underground Research Facility (Sanford Lab), was installed in 2012 and is located in the former Homestake Gold Mine in Lead, S.D. A South Dakota-owned facility, it is managed by the SDSTA, which reopened the mine in 2007 with $40 million in funding from the South Dakota State Legislature and a $70 million donation from philanthropist T. Denny Sanford. DOE's Office of Science supports Sanford Lab's operations; Berkeley Lab provided management and oversight of the DOE operations support of Sanford Lab for the past five years.

The LUX scientific collaboration, which is supported by the DOE and National Science Foundation (NSF), includes 20 research universities and national laboratories in the United States, the United Kingdom, and Portugal.

"The announcement of this new result from LUX raises the bar in the search for dark matter, exceeding our expectations," said Natalie Roe, Physics Division Director at Berkeley Lab. "With the successful completion of LUX, we are now focused on the success of LZ, which we hope will produce a dramatic discovery."

Major support for LUX came from the DOE Office of Science.