University of Maryland physicists helped develop and implement the calibration program that led to improvement
A new set of calibration techniques has once again dramatically improved the sensitivity of 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. Already proven to be the most sensitive dark matter detector in the world, LUX researchers improved the detector’s sensitivity by more than a factor of 20 for low-mass dark matter particles, significantly enhancing the team’s ability to look for the leading candidates for dark matter.
“We’ve spent two years scrutinizing our dark matter data and our dark matter detector, taking advantage of the most comprehensive calibration program ever carried out by an experiment of this type,” said Carter Hall, associate professor of physics at the University of Maryland. “We’ve dramatically improved our understanding of how our instrumentation would respond if it was to interact with the Milky Way galaxy’s dark matter halo.”
The new research, which is described in a paper posted to arXiv, re-examines data collected during LUX’s first three-month 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.
Hall and UMD physics graduate students Richard Knoche and Jon Balajthy helped develop and implement the calibration program. UMD alumnus Attila Dobi (B.S. ’08, Ph.D. ’14, physics), now a Chamberlain Fellow at Lawrence Berkeley National Laboratory, played a central role in data analysis.
Dark matter is thought to be the dominant form of matter in the universe. Scientists are confident in 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. Because weakly interacting massive particles (WIMPs) are thought to interact with other matter only on very rare occasions, they have yet to be detected directly.
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 tiny flash 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 a 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 vast mass ranges where dark matter particles might exist. 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 recoiling 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,” said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment. “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 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 Hall. "Tritiated methane is a convenient source of similar events, and we've now studied hundreds of thousands of its decays in LUX, giving us confidence that we won't mistake these garden-variety events for dark matter."
Balajthy and Knoche spent several years developing the technique to safely insert the tritiated methane into LUX and performed the delicate injection operation at the experimental site in South Dakota.
“We proceeded with great care because there was a concern that the radioactive gas would be difficult to remove from the detector,” said Hall. “If that happened, it might mean that LUX would no longer be capable of looking for dark matter. The careful experiments carried out in our labs in Maryland convinced us, our collaborators and the management team that the operation could be safely performed without harmful consequences to the experiment.”
A companion paper, also posted to arXiv, details the tritiated methane data from LUX data and the results from the demonstration experiments conducted by Knoche and Balajthy.
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 improvementscoupled 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.
“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.”
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 ⅓ 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.
“The innovations of the LUX experiment form the foundation for the LZ experiment, which is planned to achieve over 100 times the sensitivity of LUX. The LZ experiment is so sensitive that it should begin to detect a type of neutrino originating in the sun that even Ray Davis' Nobel Prize-winning experiment at the Homestake mine was unable to detect,” according to LZ spokesperson Harry Nelson from the University of California, Santa Barbara.
This work was supported by the U.S. Department of Energy and National Science Foundation. The content of this article does not necessarily reflect the views of these organizations.
The research paper, “Improved WIMP scattering limits from the LUX experiment,” by LUX Collaboration, was posted on arXiv on Dec. 11, 2015.
The research paper, “Tritium calibration of the LUX dark matter experiment,” by LUX Collaboration, was posted on arXiv on Dec. 10, 2015.
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About the Sanford Underground Research Facility
The Sanford Underground Research Facility’s mission is to enable compelling underground, interdisciplinary research in a safe work environment and to inspire our next generation through science, technology, engineering, and math education. The Sanford Lab is a South Dakota-owned facility. Homestake Mining Co. donated its gold mine in Lead to the South Dakota Science and Technology Authority (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. The U.S. Department of Energy supports Sanford Lab’s operations.
The LUX scientific collaboration, which is supported by the U.S. Department of Energy and National Science Foundation, includes 19 research universities and national laboratories in the United States, United Kingdom and Portugal. The University of Maryland is one of them.
About the College of Computer, Mathematical, and Natural Sciences
The College of Computer, Mathematical, and Natural Sciences at the University of Maryland educates more than 7,000 future scientific leaders in its undergraduate and graduate programs each year. The college's 10 departments and more than a dozen interdisciplinary research centers foster scientific discovery with annual sponsored research funding exceeding $150 million.