Improved LUX sensitivity helps redefine dark matter search parameters

Scientists working on the Large Underground Xenon (LUX) dark matter experiment are closer to ruling out the possibility of dark matter detections at low-mass ranges. The new findings come after researchers enhanced LUX’s ability to look for WIMPS, or weakly interacting massive particles, and thereafter reexamined data collected during LUX’s first run in 2013.

“We have long thought that LUX should have good sensitivity to low-mass WIMPs, but only with this new analysis are we for the first time fully taking advantage of that,” said LUX co-founder Tom Shutt from the Department of Energy’s SLAC National Accelerator Laboratory. “SLAC has played an instrumental role in the analysis, both in terms of fully understanding the detector signals and in carefully calculating the detector’s sensitivity to dark matter.”

Dark matter is thought to be the dominant form of matter in the universe. Scientists know it exists because its gravity affects the way galaxies rotate and the way light gets bent as it travels through the universe. Because WIMPs are thought to interact with other matter only on very rare occasions, they have so far evaded detection.

LUX consists of one-third of a ton of liquid xenon surrounded by sensitive light detectors, and is designed to detect collisions of dark matter particles with xenon atoms. When this happens, the xenon will recoil and emit a faint flash of light that is detected by the light sensors. The detector’s location at Sanford Lab beneath a mile of rock helps to shield it from cosmic rays and other radiation that would interfere with a dark matter signal.

LUX has yet to detect a dark matter signal, but its exquisite sensitivity has allowed scientists to rule out a vast range of properties WIMPs could have potentially had. New calibration techniques described in a paper submitted to Physical Review Letters increase that sensitivity even further, particularly for low WIMP masses.

One calibration technique used neutrons as stand-ins for dark matter particles. Bouncing neutrons off the xenon atoms allows scientists to quantify how the LUX detector responds to the recoiling process. “It’s like a giant game of pool with a neutron as the cue ball and the xenon atoms as the stripes and solids,” said LUX co-spokesman Rick Gaitskell from Brown University. “We can track the neutron to deduce the details of the xenon recoil, and calibrate the response of LUX better than anything previously possible.”

The nature of the interaction between neutrons and xenon atoms is thought to be very similar to the interaction between dark matter and xenon. “It’s just that dark matter particles interact very much more weakly—about a million-million-million-million times more weakly,” Gaitskell said.

The neutron experiments help to calibrate the detector for interactions with the xenon nucleus. But LUX scientists have also calibrated the detector’s response to the deposition of small amounts of energy by struck atomic electrons. That’s done by injecting tritiated methane—a radioactive gas—into the detector.

“In a typical science run, most of what LUX sees are background electron recoil events,” said Professor Carter Hall of the University of Maryland. “Tritiated methane is a convenient source of similar events, and we’ve now studied hundreds of thousands of its decays in LUX. This gives us confidence that we won’t mistake these garden-variety events for dark matter.”

Another radioactive gas, krypton, was injected to help scientists distinguish between signals produced by ambient radioactivity and a potential dark matter signal.

“The krypton mixes uniformly in the liquid xenon and emits radiation with a known, specific energy, but then quickly decays away to a stable, non-radioactive isotope,” said Dan McKinsey, a University of California Berkeley physics professor and co-spokesperson for LUX who is also an affiliate with Lawrence Berkeley National Laboratory. “By measuring the light and charge produced by these krypton events throughout the liquid xenon, we can flat-field the detector’s response, allowing better separation of dark matter events from natural radioactivity.”

LUX improvements, coupled to the advanced computer simulations at Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing Center (NERSC) and Brown University’s Center for Computation and Visualization (CCV), have allowed scientists to test additional particle models of dark matter that now can be excluded from the search.

Planning for the next-generation dark matter experiment at Sanford Lab is already under way. In late 2016 LUX will be decommissioned to make way for a new, much larger xenon detector, known as the LUX-ZEPLIN (LZ) experiment. Compared to LUX’s third of a ton of liquid xenon, LZ would have a 10-ton liquid xenon target, which will fit inside the same 72,000-gallon tank of pure water used by LUX.

“And so the search continues,” McKinsey said. “LUX is once again in search mode at Sanford Lab. The latest run began in late 2014 and is expected to continue until June 2016. This run will represent an increase in exposure of more than four times compared to our previous 2013 run. We will be very excited to see if any dark matter particles have shown themselves in the new data.”