New results from experimental facility deepen understanding of dark matter

LEAD, S.D. – The Large Underground Xenon (LUX) dark matter experiment, which operates nearly a mile underground at the Sanford Underground Research Facility (SURF) in the Black Hills of South Dakota, has already proven itself to be the most sensitive dark matter detector in the world. Now, a new set of calibration techniques employed by LUX scientists has further improved its sensitivity.

LUX researchers, including several from Lawrence Livermore National Laboratory’s (LLNL) Rare Event Detection Group, are looking for WIMPs, weakly interacting massive particles, which are among the leading candidates for dark matter.

LLNL is one of the founding members of the LUX experiment, and LLNL researchers have participated in LUX and its predecessor experiment (XENON-10) since 2004.

"It is vital that we continue to push the capabilities of our detector in the search for the elusive dark matter particles," said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment. "We have improved the sensitivity of LUX by more than a factor of 20 for low-mass dark matter particles, significantly enhancing our ability to look for WIMPs."

The new research is described in a paper submitted to Physical Review Letters and posted to ArXiv. The work re-examines data collected during LUX’s first experimental run in 2013, and helps to rule out the possibility of dark matter detections at low-mass ranges where other experiments had previously reported potential detections.

"The latest LUX science results are a re-analysis of data obtained over three months in 2013," said LLNL principal investigator and physicist Adam Bernstein. "The first analysis of that data was published in 2014, and since then we have expanded our understanding of the detector response through a combination of low-energy nuclear recoil measurements, low-energy electron recoil measurements and an improved understanding of our background in the low-energy recoil regime where dark matter interactions are likely to appear.

"This combination of improvements enabled us to increase our sensitivity to low-mass WIMPs by upward of two orders of magnitude. LUX is currently in a longer science run lasting 300 live days, scheduled for completion by this July," Bernstein added.

Dark matter is thought to be the dominant form of matter in the universe. Scientists are confident in its existence because its gravitational effects can be seen in the rotation of galaxies and in the way light bends as it travels through the universe. Because WIMPs are thought to interact with other matter only on very rare occasions, they have yet to be detected directly.

"We have looked for dark matter particles during the experiment's first three-month run, but are exploiting new calibration techniques that do a better job of pinning down how they would appear to our detector," said Alastair Currie of  Imperial College London. "These calibrations have deepened our understanding of the response of xenon to dark matter, and to backgrounds. This allows us to search, with improved confidence, for particles that we hadn't previously known would be visible to LUX."

Bernstein and other LLNL researchers have taken part in initial science planning and experimental design for LUX. Physicist Peter Sorensen, formerly with LLNL and now at Lawrence Berkeley National Laboratory, spent many months with on-site assembly and commissioning, and has made key contributions to the study of the low-mass WIMP signal.

Physicist Kareem Kazkaz, who works in the LLNL Rare Event Detection Group, created the LUXSim simulation framework, which has been used throughout the collaboration to understand detector response and increase the team’s understanding of signal backgrounds and how the liquid xenon medium responds to incident radiation.

More recently, LLNL graduate scholar Brian Lenardo has served as the deputy science coordination manager and has been an integral member of the team studying the light and charge yield of nuclear recoils within the active volume. Joining LLNL in September, postdoctoral fellow Jingke Xu has organized a sub-group focused on events at the single electron quantum limit of detector sensitivity.

LUX consists of a third of a ton of liquid xenon surrounded with sensitive light detectors. It is designed to identify the very rare occasions when a dark matter particle collides with a xenon atom inside the detector. When a collision happens, the xenon atom will recoil and emit a small burst of light, which is detected by LUX’s 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 the dark matter signal.

So far, LUX hasn’t detected a dark matter signal, but its exquisite sensitivity has allowed scientists to all but rule out dark matter particles over a wide range of masses that current theories allow. These new calibrations increase that sensitivity even further.

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 recoil process.

"It is like a giant game of pool with a neutron as the cue ball and the xenon atoms as the stripes and solids," Gaitskell said. "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 also have 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 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, nonradioactive Isotope, " said Dan McKinsey, a University of California Berkeley physics professor and co-spokesperson for LUX, who also is an affiliate of 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 and Brown University’s Center for Computation and Visualization have allowed scientists to test additional particle models of dark matter that can be excluded from the search. "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 the previous 2013 run. We will be very excited to see if any dark matter particles have shown themselves in the new data."

The LUX scientific collaboration, which is supported by the DOE and National Science Foundation, includes 19 research universities and national laboratories in the United States, the United Kingdom and Portugal.

"The global search for dark matter aims to answer one of the biggest questions about the makeup of our universe. We’re proud to support the LUX collaboration and congratulate them on achieving an even greater level of sensitivity," said Mike Headley, executive director of the SDSTA.