Contributed by the LUX Collaboration
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 again dramatically improved its sensitivity.
Researchers with LUX are looking for WIMPs, weakly interacting massive particles, which are among the leading candidates for dark matter.
“The nature of dark matter is one of the most important questions facing physics today, and this effort by the LUX collaboration is the most sensitive experiment for direct detection of WIMPs,” said Mani Tripathi, professor of physics at UC Davis and one of the founding principal investigators of LUX.
The LUX team at UC Davis consists of postdoctoral researchers Aaron Manalaysay and Scott Stephenson, and graduate students Sergey Uvarov, Brian Lenardo, James Morad, and Jacob Cutter. Several undergraduate students are involved in this research, including Eric Emdee, Thomas Kurty, Nathaniel Nunez, Jack Zilinkas, and Megha Jain.
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 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.
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 WIMPs are thought to interact with other matter only on very rare occasions, they have yet to be detected directly.
Video: 4850 feet below: The hunt for dark matter (Science Friday)
Giant tank of liquid xenon
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.
Calibrating with neutrons
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. Understanding this process in depth is aided by the Noble Element Simulation Technique (NEST), which is a software package developed primarily at UC Davis.
“Calibration of the LUX detector with neutrons has helped to inform our physics models describing the response that liquid xenon will have to WIMPs,” said Brian Lenardo, who led the effort to build these physics models that are used in NEST, and which are integral to the dark-matter results of LUX.
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 give 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 UC 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.
The LUX group at UC Davis operates the on-site computing cluster, which is a powerful supercomputer at the lab in South Dakota, and contributes to the processing of data arriving from the LUX detector. LUX generates enormous amounts of data each second, which has to be shared among various computer clusters around the world and processed to extract meaningful dark-matter results. “The challenging data-processing process involves code written by numerous people run on computing clusters at various institutions,” said James Morad, who served as LUX’s data processing manager.
Searching and new experiments
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.
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 (DOE) supports Sanford Lab’s operations.
The LUX scientific collaboration, which is supported by the DOE and National Science Foundation (NSF), includes 19 research universities and national laboratories in the United States, the United Kingdom, and Portugal.
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 Harry Nelson from the University of California Santa Barbara, spokesperson for LZ.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit the Office of Science website at science.energy.gov.
For more information, please visit the Sanford Lab website at www.sanfordlab.org.
LUX Experiment Web Site: http://luxdarkmatter.org
Photos of LUX Experiment: http://luxdarkmatter.org/LUX_dark_matter/Photos.html, http://pics.sanfordlab.org/lux2015pr