Next-Gen Dark Matter Detector in a Race to Finish Line
Mile-deep U.S.-based experiment is on a fast track to help solve science mystery
The race is on to build the most sensitive U.S.-based experiment designed to directly detect dark matter particles. Department of Energy officials today formally approved a key construction milestone that will propel the project toward its April 2020 goal for completion.
The LUX-ZEPLIN (LZ) experiment, which will be built nearly a mile underground at the Sanford Underground Research Facility (SURF) in Lead, S.D., is considered one of the best bets yet to determine whether theorized dark matter particles known as WIMPs (weakly interacting massive particles) actually exist. The LZ collaboration has about 220 participating scientists and engineers who represent 38 institutions around the globe.
On Feb. 9, the project passed a DOE review and approval stage known as Critical Decision 3 (CD-3), which accepts the final design and formally launches construction.
The fast-moving schedule for LZ will help the U.S. stay competitive with similar next-gen dark matter direct-detection experiments planned in Italy and China. “The science is highly compelling, so it’s being pursued by physicists all over the world,” said Carmen Carmona, professor of physics at Penn State and member of the LZ collaboration. “It’s a friendly and healthy competition, with a major discovery possibly at stake.”
The nature of dark matter—which physicists describe as the invisible component or so-called “missing mass” in the universe that would explain the faster-than-expected spins of galaxies, and their motion in clusters observed across the universe—has eluded scientists since its existence was deduced through calculations by Swiss astronomer Fritz Zwicky in 1933.
The quest to find out what dark matter is made of, or to learn whether it can be explained by tweaking the known laws of physics in new ways, is considered one of the most pressing questions in particle physics.
Luiz de Viveiros, also professor of physics at Penn State and member of the LZ collaboration, noted that while WIMPs are the primary target for LZ and its competitors, LZ’s explorations into uncharted territory could lead to a variety of surprising discoveries. “People are developing all sorts of models to explain dark matter,” he said. “LZ is optimized to observe a heavy WIMP, but it’s sensitive to some less-conventional scenarios as well. It can also search for other exotic particles and rare processes.”
Successive generations of experiments have evolved to provide extreme sensitivity in the search that will at least rule out some of the likely candidates and hiding spots for dark matter, or may lead to a discovery.
LZ will be at least 50 times more sensitive to finding signals from dark matter particles than its predecessor, the Large Underground Xenon experiment (LUX), which was removed from SURF last year to make way for LZ. The new experiment will use 10 metric tons of ultra-purified liquid xenon, to tease out possible dark matter signals. Xenon, in its gas form, is one of the rarest elements in Earth’s atmosphere.
All of the components for LZ are painstakingly measured for naturally occurring radiation levels to account for possible false signals coming from the components themselves. A dust-filtering cleanroom is being prepared for LZ’s assembly and a radon-reduction building is under construction at the South Dakota site—radon is a naturally occurring radioactive gas that could interfere with dark matter detection.
Professors Carmona and de Viveiros are leading the LZ team at Penn State University, which includes efforts to measure trace radon contamination levels present in the LZ components, and to develop real-time radon monitoring technologies suitable for use in liquid xenon temperatures. These steps are necessary to remove background signals as much as possible.
A planned upgrade to the current XENON1T experiment at National Institute for Nuclear Physics’ Gran Sasso Laboratory (the XENONnT experiment) in Italy, and China’s plans to advance the work on PandaX-II, are also slated to be leading-edge underground experiments that will use liquid xenon as the medium to seek out a dark matter signal. Both of these projects are expected to have a similar schedule and scale to LZ, though LZ participants are aiming to achieve a higher sensitivity to dark matter than these other contenders.
LZ is designed so that if a dark matter particle collides with a xenon atom, it will produce a prompt flash of light followed by a second flash of light when the electrons produced in the liquid xenon chamber drift to its top. The light pulses, picked up by a series of about 500 light-amplifying photomultiplier tubes (PMTs) lining the massive tank—over four times more than were installed in LUX—will carry the telltale fingerprint of the particles that created them.
“We are looking forward to seeing everything come together after a long period of design and planning,” Prof. Carmona said. Preparations of the cavern where LZ will be housed is underway at SURF, she added, and onsite assembly and installation will begin in 2018.
For more information about LZ and the LZ collaboration, visit: http://lz.lbl.gov/. Major support for LZ comes from the DOE Office of Science’s Office of High Energy Physics, South Dakota Science and Technology Authority, the UK’s Science & Technology Facilities Council, and by collaboration members in South Korea and Portugal.
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. For more information, please visit the Sanford Lab website at http://www.sanfordlab.org.
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.