Last week, the Large Underground Xenon (LUX) collaboration announced a whole new level of sensitivity for its dark matter experiment. Although no dark matter particles were found, LUX’s sensitivity far exceeded the goals for the project. The results give researchers confidence that if a particle had interacted with the detector’s xenon target, they almost certainly would have seen it.
“It would have been marvelous if the improved sensitivity had also delivered a clear dark matter signal. However, what we have observed is consistent with background alone,” said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for LUX.
The new results allow scientists to eliminate many potential models for dark matter particles, offering critical guidance for the next generation of dark matter experiments. The final results were announced at the Identification of Dark Matter 2016 conference and signaled the completion of a 300-live-day search that ended in May.
During a 20-month run, the LUX team incorporated unique calibration measures to search a wide swath of potential parameter space for dark matter particles called WIMPs, or weakly interacting massive particles.
“These careful background-reduction techniques and precision calibrations and modeling, enabled us to probe dark matter candidates that would produce signals of only a few events per century in a kilogram of xenon,” said Aaron Manalaysay, the Analysis Working Group coordinator for LUX and a research scientist from UC Davis, who presented the new results in Sheffield, UK.
With the completion of its final run, LUX is preparing for decommissioning this fall. But before that, the LUX team plans to use the detector to continue calibrating and testing backgrounds in preparation for the next generation dark matter detector, LUX-ZEPLIN (LZ).
“The main driver behind this campaign of calibrations is to test new techniques or improve on existing techniques, which will be used for LZ,” said Simon Fiorucci, a physicist at Lawrence Berkeley National Laboratory and science coordination manager for the experiment. LUX has sufficient size, low-enough background and a known response that can tell researchers if the techniques will work.
Fiorucci said some interesting science also can come out of some of these tests. For example, the neutron generator studies done in June and July could further improve understanding of the xenon response to WIMP interactions at extremely low energy. “This would be a boon to LZ, LUX and the entire field of dark matter,” he said.
The LZ team also plans to measure the intrinsic radioactivity of a liquid scintillator mix that will be used with LZ and requires an extremely quiet environment. The scintillator will replace LUX inside the high-purity water tank.
“This critical piece of information will tell LZ whether their background is good enough for the outer detector to perform as expected and, if not, where they should focus their efforts to make it so,” Fiorucci said.
The tests will run through January.
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The Sanford Underground Research Facility in Lead, South Dakota, advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.
The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.
Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.
In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.
In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.
The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.
LUX/Dark matter experiment at SURF
In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe. The Majorana Demonstrator experiment, also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.
Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.