20-month U.S. experiment narrows search for dark matter particle

An 20-month experiment by Large Underground Xenon, or LUX, has come to an end, narrowing the search for, but failing to find, an elusive particle that should be everywhere.

The experiment operated beneath a mile, or 1.6 kilometer, of rock at the Sanford Underground Research Facility in the former Homestake gold mine in the Black Hills of South Dakota. Its mission: to detect so-called weakly interactive massive particles, or WIMPS, that some physicists think comprise the mysterious dark matter that makes up 85 percent of all the matter in the universe.

If the WIMP hypothesis is correct, billions of these particles pass through our bodies every second, as well as Earth and everything on it. But because WIMPs interact so weakly with ordinary matter, this ghostly traverse goes entirely unnoticed.

Despite the LUX team' s non-discovery, the researchers have narrowed the mass-range in which the putative particle can exist, focusing future searches within that range.

"The extra experiment gave us more sensitivity in searching for high-mass dark matter particles," Daniel McKinsey, a University of California, Berkeley, professor of physics and senior faculty scientist at Lawrence Berkeley National Laboratory, and co-spokesperson for LUX, was quoted as saying by a news release from the school on Thursday. "Many theorists think that the dark matter mass will be high, around a thousand times the mass of a proton."

The experiment provided about four times better sensitivity to high-mass particles than earlier underground experiments that began in 2013. LUX' s sensitivity far exceeded the original goals for the project, according to McKinsey, which 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. That enables them to eliminate many potential models for dark matter particles, offering guidance for the next generation of dark matter experiments.

Key to the improved sensitivity was a calibration technique that allowed the researchers to remove sources of noise caused by electrons building up on the inner Teflon coating of the tank holding a third-of-a-ton of cooled liquid xenon. If a WIMP collided with of a xenon atom within the tank, sensors inside would detect the tiny flash of light and electrical charge created.

"Teflon is a terrific reflector of light, but it builds up electrons, which distort the electric field we use to detect the effects of dark matter," McKinsey said. "So we relied on our calibrations to compensate. That was fairly tricky and took a lot of effort."

The data analysis alone took more than 1,000 computer nodes at Brown University' s Center for Computation and Visualization (CCV) in Rhode Island and the advanced computer simulations at Lawrence Berkeley National Laboratory' s National Energy Research Scientific Computing Center (NERSC) in northern California.

While the experiment eliminated a large swath of mass ranges and interaction-coupling strengths where WIMPs might exist, the WIMP model itself "remains alive and viable," said Rick Gaitskell, a professor of physics at Brown University and co-spokesperson for the LUX experiment. "LUX was racing over the last three years to get first evidence for a dark matter signal. We will now have to wait and see if the new run this year at the Large Hadron Collider at CERN will show evidence of dark matter particles, or if the discovery occurs in the next generation of larger direct detectors."

CERN, short for European Organization for Nuclear Research, is based in Geneva, Switzerland.

First constructed in 2011, LUX completed its final search in May, and now will be upgraded to a more sensitive experiment called LUX-ZEPLIN that draws in researchers who built a similar underground dark matter detector in the United Kingdom called ZEPLIN. Endit