LEAD (Oct. 30, 2013) - Blazing the path to illuminate the world, a new experiment being conducted at Sanford's deep underground research laboratory has proven the dark matter detector is the most sensitive of its kind. Mines physicists have played a significant role in the groundbreaking research from the beginning.
Collaborating scientists from around the world announced Wednesday that the first three-month run of experiments has proven the Large Underground Xenon (LUX) detector to be 20 times more sensitive than similar experimental detectors, allowing scientists to exclude other particle interactions and establish a baseline for future dark matter detection.
Though LUX has been online for less than a year, one of the LUX goals was to figure out how to build an even larger detector for the next generation of dark matter experiments. Designs for a 4-ton version of LUX, call LZ, which will have a factor of 1,000 times more sensitivity, have been submitted.
"LUX is blazing the path to illuminate the nature of dark matter," said Brown University physicist Rick Gaitskell, who is co-spokesperson for LUX along with physicist Dan McKinsey of Yale University.
The LUX scientific collaboration, which is supported by the National Science Foundation and Department of Energy, includes 17 research universities and national laboratories and 100 scientists in the United States, the United Kingdom and Portugal. South Dakota School of Mines & Technology physicists were initially tasked with characterizing and calibrating photomultiplier tubes (PMTs) to identify dark matter.
A School of Mines contingent at Sanford for the Oct. 30 announcement included Xinhua Bai, Alberto Lemut and Luke Corwin, physics department researchers; Duane Hrncir, provost; and Mark Hanhardt, LUX operations manager and current physics Ph.D. student.
"There is significant collaboration with Mines and enormous advantages to working with local students - and for the students, they have this incredible opportunity in their own backyard. It's literally on their doorstep, and the science is world-class. We have 100 investigators including Xinhua Bai, who is a very smart researcher. ... I've enjoyed working with him," said Gaitskell, noting Bai's experience. Bai has dedicated much of his research efforts to muons, one major cause of the background.
Dark matter, so far observed only by its gravitational effects on galaxies and clusters of galaxies, is the predominant form of matter in the universe.
The possibility of discovering elusive dark matter, and the proximity of the School of Mines to Sanford, is what attracted Bai to the Black Hills in 2009 from the University of Delaware. With support from former colleagues in the South Pole Air Shower Experiment (SPASE-2) and Antarctica Muon and Neutrino Array (AMANDA) project, Bai built his own astroparticle physics laboratory from scratch at the School of Mines and formed a team of graduate-level researchers.
This fall, the state's first Ph.D. in physics was launched at the School of Mines and University of South Dakota. Mines doctoral students will play a significant role in Sanford's research activities.
Scientists began collecting data in early 2013 in hopes of identifying dark matter.
The School of Mines calibrated 20 photomultiplier tubes (PMTs), which are installed inside the tank and which researchers hope will help identify the presence of elusive dark matter particles by essentially tagging and eliminating more everyday particles within the Standard Model, a well-established particle physics model that describes the physics of the visible portion of the universe.
SDSM&T researchers also developed a muon tagging system for the LUX project. Muon is an elementary particle that exists everywhere on the earth's surface. Deep penetrating, some high-energy muons can break through 4,850 feet of rock and reach Davis Cavern where the LUX detector is located.
Mines students also installed Tyvek in the tank before it was sealed and filled with water and finished a dust characterization system that is designed to measure the inherent charge on dust particles present at the facility, which has been used in the LUX cleanroom space on the surface.
While a Mines master's student, Hanhardt calibrated and characterized the 10-inch PMTs in Bai's campus laboratory. After earning his M.S. in 2011, Handhardt went on to a similar position at the Soudan Underground Lab in Soudan, Minn., but recently returned to enroll in SDSMT's new Ph.D. program.
"This announcement is very exciting. It really is the culmination of lots of people's work and international collaboration," Hanhardt said.
"The idea that I can be involved in this cutting-edge research and pursue my Ph.D. is a dream come true and the whole reason I came back to South Dakota. USD, the Department of Energy, NSF, SDSM&T and the lab all come together to make it work and to make it possible for me to do work. This lab is unique. It's deep and dedicated to science. In the next decade or so, this is where big science is going to happen," Hanhardt said.
Tom Shutt of Case Western Reserve University, former LUX co-spokesperson and current LZ spokesperson, agrees. With the next generation detector LZ being 1,000 times more sensitive even than LUX within a few years, the Sanford dark matter project "will be the experiment to beat for most of the next decade or more."
Though WIMPS, or weakly interacting massive particles, which are the leading theoretical candidates for dark matter, were not detected, the increased sensitivity will allow more interacting "events" to be detected, including the possibility of WIMPS.
WIMPs rarely interact with ordinary matter except through gravity. The mass of WIMPs is unknown, but theories and results from other experiments suggest a number of possibilities.
LUX has a peak sensitivity at a WIMP mass of 33 GeV/c2, with a sensitivity limit three times better than any previous experiment. LUX also has a sensitivity that is more than 20 times better than previous experiments for low-mass WIMPs, whose possible detection has been suggested by other experiments. Three candidate low-mass WIMP events recently reported in ultra-cold silicon detectors would have produced more than 1,600 events in LUX's much larger detector, or one every 80 minutes in the recent run. No such signals were seen.
"This is only the beginning for LUX," Yale's McKinsey said. "Now that we understand the instrument and its backgrounds, we will continue to take data, testing for more and more elusive candidates for dark matter."
In both theory and practice, collisions between WIMPs and normal matter are rare and extremely difficult to detect, especially because a constant rain of cosmic radiation from space can drown out the faint signals. That's why LUX is searching for WIMPs 4,850 feet underground in the Sanford Lab, where few cosmic ray particles can penetrate. The detector is further protected from background radiation from the surrounding rock by immersion in a tank of ultra-pure water.
"This supremely quiet environment substantially improves our ability to see WIMPs scattering with xenon nuclei," said Gaitskell.
At the heart of the experiment is a 6-foot-tall titanium tank filled with almost a third of a ton of liquid xenon, cooled to minus 150 degrees Fahrenheit. If a WIMP strikes a xenon atom it recoils from other xenon atoms and emits photons (light) and electrons. The electrons are drawn upward by an electrical field and interact with a thin layer of xenon gas at the top of the tank, releasing more photons.
Light detectors in the top and bottom of the tank are each capable of detecting a single photon, so the locations of the two photon signals - one at the collision point, the other at the top of the tank - can be pinpointed to within a few millimeters. The energy of the interaction can be precisely measured from the brightness of the signals.
"LUX is a complex instrument," said McKinsey, "but it ensures that each WIMP event's unique signature of position and energy will be precisely recorded."
LUX's biggest advantage as a dark matter detector is its size, a large xenon target whose outer regions further shield the interior from gamma rays and neutrons. Installed in the Sanford Lab in the summer of 2012, the experiment was filled with liquid xenon in February, and its first run of three months was conducted this spring and summer, followed by intensive analysis of the data. The dark matter search will continue through the next two years.
"The universe's mysterious dark sector presents us with two of the most thrilling challenges in all of physics," says Saul Perlmutter of DOE's Lawrence Berkeley National Laboratory (Berkeley Lab), a winner of the 2011 Nobel Prize in Physics for discovering the accelerating expansion of the universe. "We call it the dark sector precisely because we don't know what accounts for most of the energy and mass in the universe. Dark energy is one challenge, and as for the other, the LUX experiment's first data now take the lead in the hunt for the dark matter component of the dark sector."
South Dakota Gov. Dennis Daugaard said his state is proud to play a role in this important research. Homestake Mining Co. donated its gold mine in Lead to the South Dakota Science and Technology Authority, which reopened it in 2007 with funding from the state Legislature and a $70 million donation from philanthropist T. Denny Sanford.
"We congratulate the LUX researchers, and we look forward to working with dark matter scientists and other partners in the years to come," Daugaard said.
The LUX announcement is a major step forward for the Sanford Lab's science program, which Laboratory Director Mike Headley points out has its roots in a famous physics experiment installed in the same experiment hall in the 1960s. "These are the first physics results achieved at Homestake since the Ray Davis solar neutrino experiment, which earned him a Nobel Prize for Physics," Headley said. "I'm very proud of our staff's work to help LUX reach this major milestone."
Planning for the next-generation dark matter experiment at the Sanford Lab is already under way. Compared to LUX's third of a ton of liquid xenon, the LUX-ZEPLIN, or LZ, experiment would have a 7-ton liquid xenon target inside the same 72,000-gallon tank of pure water used by LUX. With the next generation LZ a thousand times more sensitive than the LUX detector, Shutt said, "It will just begin to see an irreducible background of neutrinos that may ultimately set the limit to our ability to measure dark matter."
LUX and LZ are among 14 active research groups at the Sanford Lab, including the Majorana Demonstrator collaboration, which is looking for one of the rarest forms of radioactive decay in an experiment hall adjacent to LUX. Other teams of researchers are planning experiments in physics, geology and biology that could extend the future of the lab for decades.
"LUX's first result is a great reward for the Department of Energy's leadership of the Sanford Underground Research Facility," said Berkeley Lab physicist Kevin Lesko, who oversees operations at the Sanford Lab for DOE's Office of Science. "The collaborations and especially the staff of the Sanford Lab are to be commended for their determination in pursuing research, and especially for creating a deep underground facility for the worldwide physics community that provides US scientists with opportunities for continued leadership in their pursuit of the most compelling physics questions."
View photos of the LUX detector 4,850 feet below the Earth's surface: http://www.flickr.com/photos/sdsmt/sets/72157637137264495/
Founded in 1885, the South Dakota School of Mines & Technology is a science and engineering research university located in Rapid City, S.D., offering bachelor's, master's and doctoral degrees. The university enrolls 2,640 students from 45 states and 37 countries, with a student-to-faculty ratio of 14:1. The average starting salary for graduates is $62,400 with a 98 percent placement rate. Find us online at www.sdsmt.edu, on Facebook at https://www.facebook.com/sdsmt and on Twitter at https://twitter.com/sdsmt.
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 www.science.energy.gov.
The Sanford Underground Research Facility's mission is to enable safe and compelling underground research and to foster transformational science education. For more information, please visit the Sanford Lab website at www.sanfordlab.org.