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 (DOE) officials today formally approved a key construction milestone that will propel the project toward its April 2020 goal for completion.
Lawrence Livermore National Laboratory (LLNL) is a founding member of the LZ collaboration -- and its predecessor, the Large Underground Xenon experiment. Four researchers from LLNL are part of the 220-scientist team from 10 countries that is working on the project.
The LUX-ZEPLIN (LZ) experiment, which will be built nearly a mile underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, is considered one of the best bets yet to determine whether theorized dark matter particles known as WIMPs (weakly interacting massive particles) actually exist. There are other dark matter candidates, too, such as “axions” or “sterile neutrinos,” which other experiments are better suited to root or rule out.
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.
On Feb. 8, 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.
“We will try to go as fast as we can to have everything completed by April 2020,” said Murdock “Gil” Gilchriese, LZ project director and a physicist at DOE's Lawrence Berkeley National Laboratory, the lead lab for the project. “We got a very strong endorsement to go fast and to be first.” The LZ collaboration now has about 220 participating scientists and engineers who represent 38 institutions around the globe.
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 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.
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.
“The science is highly compelling, so it’s being pursued by physicists all over the world,” said Carter Hall, the spokesperson for the LZ collaboration and an associate professor of physics at the University of Maryland. “It's a friendly and healthy competition, with a major discovery possibly at stake.”
A planned upgrade to the current XENON1T experiment at Italy's Gran Sasso National Laboratory (the XENONnT experiment), and China's plans to advance the work on PandaX-II, also are slated to be leading-edge underground experiments that will use liquid xenon as the medium to seek out a dark matter signal. Both 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.
Hall noted that while WIMPs are a 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 also can search for other exotic particles and rare processes.”
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 tubes lining the massive tank -- four times more than were installed in LUX -- will carry the telltale fingerprint of the particles that created them.
Atoms of xenon, a component in some vehicle headlights and other types of lighting, emit flashes of light and electrical pulses in particle interactions that LZ is designed to measure using hundreds of light-sensing devices known as photomultiplier tubes.
Adam Bernstein, the Rare Event Detection Group leader at LLNL, is the head of the LLNL LZ team, consisting of Bernstein, staff physicist Kareem Kazkaz, postdoctoral fellow Jingke Xu and graduate student Brian Lenardo.
Livermore’s contributions to the LZ program include using a small dual-phase test bed at LLNL to study the detector response to WIMP-like interactions at its low energy limit -- corresponding to sensitivity to single electronics generated in the xenon target.
Understanding of both the signal characteristics and noise properties of the detector in this crucial region will permit LZ to extend its sensitivity beyond its current baseline.
LZ also is adopting a detector simulation architecture known as BACCARAT, developed by Kazkaz at LLNL. BACCARAT will be used to provide detailed predictions of detector sensitivity to the WIMP signal prior to deployment, and in the data and simulation comparisons that are at the heart of the WIMP sensitivity analysis.
All of the components for LZ are painstakingly measured for naturally occurring radiation levels to account for possible false signals coming from the components. A dust-filtering clean room 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. These steps are necessary to remove background signals as much as possible.
The vessels that will surround the liquid xenon, which are the responsibility of the U.K. participants of the collaboration, are now being assembled in Italy. They will be built with the world's most ultra-pure titanium to further reduce background noise.
To ensure unwanted particles are not misread as dark matter signals, LZ's liquid xenon chamber will be surrounded by another liquid-filled tank and a separate array of photomultiplier tubes that can measure other particles and largely veto false signals.
Collaborators at Fermi National Accelerator Laboratory are implementing key parts of the critical system to handle and cool the purified xenon, and Brookhaven National Laboratory is handling the production of another very pure liquid, known as a scintillator fluid, that will surround the xenon detector vessels.
The clean rooms will be in place by June, Gilchriese said, and preparation of the cavern where LZ will be housed is underway at SURF. On-site assembly and installation will begin in 2018, he added, and all of the xenon needed for the project has either been delivered or is under contract. Xenon gas, which is costly to produce, is used in lighting, medical imaging and anesthesia, space-vehicle propulsion systems, and the electronics industry.
Major support for LUX 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.