Editor’s note: A Fermilab group led by Hugh Lippincott, a Wilson fellow at Fermilab, and Eric Dahl, a joint appointee and assistant professor at Northwestern University, is responsible for implementing key parts of the critical systems that handle the xenon in the LUX-ZEPLIN detector. These include the heat exchange system that allows the entire 10-ton xenon target to be purified every 2.3 days, as well as the controls system that safeguards the greater than $10 million xenon payload in emergency scenarios. View the original Berkeley Lab press release on their news site.
The race is on to build the most sensitive U.S.-based experiment designed to directly detect dark matter particles. Department of Energy officials have formally approved a key construction milestone that will propel the project toward its April 2020 goal for completion.
A production prototype of highly purified, gadolinium-doped scintillator fluid, viewed under ultraviolet light. Scintillator fluid will surround LZ’s xenon tank and will help scientists veto the background “noise” of unwanted particle signals. Photo: Brookhaven National Laboratory
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. There are other dark matter candidates, too, such as “axions” or “sterile neutrinos,” which other experiments are better suited to root out 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.
This image shows a cutaway rendering of the LUX-ZEPLIN (LZ) detector that will search for dark matter nearly a mile below ground. An array of detectors, known as photomultiplier tubes, at the top and bottom of the liquid xenon tank are designed to pick up particle signals. Image: Matt Hoff/Berkeley Lab
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.
“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 the DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), 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 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.
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.
The underground home of LZ and its supporting systems are shown in this computerized rendering. Image: Matt Hoff/Berkeley Lab
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.”
This chart shows the sensitivity limits (solid-line curves) of various experiments searching for signs of theoretical dark matter particles known as WIMPs, with LZ (green dashed line) set to expand the search range. Image: Snowmass report, 2013
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.
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 can also search for other exotic particles and rare processes.”
Light-amplifying devices known as photomultiplier tubes (PMTs), developed for use in the LUX-ZEPLIN (LZ) dark matter-hunting experiment, are prepared for a test at Brown University. This test bed, dubbed PATRIC, will be used to test over 600 PMTs in conditions simulating the temperature and pressure of the liquid xenon that will be used for LZ. Photo: Brown University
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—over four times more than were installed in LUX—will carry the telltale fingerprint of the particles that created them.
Inside LZ: When a theorized dark matter particle known as a WIMP collides with a xenon atom, the xenon atom emits a flash of light (gold) and electrons. The flash of light is detected at the top and bottom of the liquid xenon chamber. An electric field pushes the electrons to the top of the chamber, where they generate a second flash of light (red). Image: SLAC National Accelerator Laboratory
Daniel Akerib, Thomas Shutt, and Maria Elena Monzani are leading the LZ team at SLAC National Accelerator Laboratory. The SLAC effort includes a program to purify xenon for LZ by removing krypton, an element that is typically found in trace amounts with xenon after standard refinement processes.
“We have already demonstrated the purification required for LZ and are now working on ways to further purify the xenon to extend the science reach of LZ,” Akerib said.
SLAC and Berkeley Lab collaborators are also developing and testing hand-woven wire grids that draw out electrical signals produced by particle interactions in the liquid xenon tank. Full-size prototypes will be operated later this year at a SLAC test platform. “These tests are important to ensure that the grids don’t produce low-level electrical discharge when operated at high voltage, since the discharge could swamp a faint signal from dark matter,” said Shutt.
Assembly of the prototype for the LZ detector’s core, known as a time projection chamber (TPC). From left: Jeremy Mock (State University of New York/Berkeley Lab), Knut Skarpaas, and Robert Conley. Photo: SLAC National Accelerator Laboratory
Hugh Lippincott, a Wilson Fellow at Fermi National Accelerator Laboratory (Fermilab) and the physics coordinator for the LZ collaboration, said, “Alongside the effort to get the detector built and taking data as fast as we can, we’re also building up our simulation and data analysis tools so that we can understand what we’ll see when the detector turns on. We want to be ready for physics as soon as the first flash of light appears in the xenon.” Fermilab is responsible for implementing key parts of the critical system that handles, purifies, and cools the xenon.
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. These steps are necessary to remove background signals as much as possible.
A rendering of the Surface Assembly Laboratory in South Dakota where LZ components will be assembled before they are relocated underground. Image: LZ collaboration
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. Brookhaven National Laboratory is handling the production of another very pure liquid, known as a scintillator fluid, that will go into this tank.
The cleanrooms will be in place by June, Gilchriese said, and preparation of the cavern where LZ will be housed is underway at SURF. Onsite assembly and installation will begin in 2018, he added, and all of the xenon needed for the project has either already 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.
“South Dakota is proud to host the LZ experiment at SURF and to contribute 80 percent of the xenon for LZ,” said Mike Headley, executive director of the South Dakota Science and Technology Authority (SDSTA) that oversees SURF. “Our facility work is underway and we’re on track to support LZ’s timeline.”
UK scientists, who make up about one-quarter of the LZ collaboration, are contributing hardware for most subsystems. Henrique Araújo, from Imperial College London, said, “We are looking forward to seeing everything come together after a long period of design and planning.”
LZ participants conduct a quality-control inspection of photomultiplier tube bases that are being manufactured at Imperial College London. Photo: Henrique Araújo/Imperial College London
Kelly Hanzel, LZ project manager and a Berkeley Lab mechanical engineer, added, “We have an excellent collaboration and team of engineers who are dedicated to the science and success of the project.” The latest approval milestone, she said, “is probably the most significant step so far,” as it provides for the purchase of most of the major components in LZ’s supporting systems.
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.
Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel Prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.
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.
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.
As we enter the second month of Fermilab’s 50th year, we look back on Robert Wilson assuming the lab’s first directorship and the lab’s first experiment, along with other memorable milestones.
Evidence of CP violation was observed by the CDF experiment.
Feb. 5, 1999: Evidence of CP violation in neutral B mesons
On this day, CDF scientists reported evidence of CP violation in neutral B mesons — a phenomenon that could help explain why matter predominates in our universe while antimatter is nearly absent.
The E-36 experimenters gather for a group photo.
Feb. 12, 1972: First experiment
Experiment E-36, Small Angle Proton-Proton Scattering, began testing equipment in the lab’s newly achieved 100-GeV beam in the Internal Target Area on this day, marking the beginning of the lab’s experimental program. The E-36 experimenters came from the National Accelerator Laboratory, the Joint Institute for Nuclear Research (Dubna, U.S.S.R.), the University of Rochester (Rochester, New York) and Rockefeller University (New York City), making it a model of cooperation between Americans and Soviets at a time when Cold War tensions still ran high.
The DZero detector is visible at the far end of the hall behind the flags. The blue tower is the mobile counting house where all the cables from the detector are routed to. Both moved in unison with each other. Once the detector was in place, a concrete blocks were stacked to cover opening, providing shielding. Some of the blocks are visible on either side of the detector.
Feb. 14, 1992: DZero detector placed in Tevatron
Named for the section of the Tevatron ring where it was installed, the DZero detector was rolled into place on this day. By this day, the DZero scientific collaboration had grown to about 300 members. First collisions were seen three months later, and the experiment began taking data in September 1992.
The Tevatron accelerated and extracted beam to 800 billion electronvolts in February 1984.
Feb. 15, 1984: Tevatron achieves proton beam energy of 800 GeV
With the achievement of an 800-GeV operating beam energy, on this day the Tevatron regained the lead as the highest-energy fixed-target laboratory in the world. An 800-GeV ramp was set up in the Tevatron and achieved by 6:30 p.m. The Tevatron then accelerated and extracted beam to 800 GeV by 10:54 p.m. The beam marked the opening of a new energy frontier in high-energy physics.
Robert Wilson was Fermilab’s first director.
Feb. 28, 1967: Robert Wilson becomes lab director
On Feb. 28, 1967, Robert R. Wilson, director of the Laboratory for Nuclear Studies at Cornell University, agreed to direct the planned National Accelerator Laboratory. Wilson was born in Frontier, Wyoming, and spent time on his family’s cattle ranch as a child. He also immersed himself in books from his local library and experimented in his mother’s attic, a background which nurtured his independence and his love of the challenge of the frontier. He received his Ph.D. from the University of California, Berkeley, and became the youngest group leader on the Manhattan Project. He was also an avid sculptor, and his artistic sensibilities would shape Fermilab’s unique aesthetic. In 1987, on the occasion of the lab’s 20th anniversary, he would write that his “fantasy of a utopian laboratory clearly required a setting of environmental beauty, of architectural grandeur, of cultural splendor.”
Roger Dixon and Erik Ramberg (third and fourth from left) have directed Fermilab’s successful Saturday Morning Physics for more than 20 years. Now they pass the torch to Elliott McCrory (far left) and Sowjanya Gollapinni (not pictured). Suzanne Weber (second from left) has been a pillar of the program, serving as coordinator since 1990. Photo: Dan Garisto
Over the past 37 years, local students have spent their Saturday mornings not in front of a TV watching cartoons, but in front of a blackboard, learning physics.
Since its inception in 1980 by then-Fermilab Director Leon Lederman, Saturday Morning Physics has been one of the most popular outreach initiatives at Fermilab. It has seen thousands of students pass through its 10-week-long programs, each of which regularly draws around 150 participants. During the course, students are exposed to topics from the history and practice of science to quantum mechanics to neutrino research at Fermilab.
Several copycat programs as near as the University of Michigan and as far as Darmstadt, Germany, attest to the program’s success instilling an appreciation of science and understanding of physics.
Neither snow nor rain nor heat have ever stopped the classes of Saturday Morning Physics, whose website warns students that “class is never cancelled due to weather.” According to Fermilab physicists Roger Dixon and Erik Ramberg, who have been directors of Saturday Morning Physics for more than 20 years, class has been cancelled only once: during the government shutdown of 2013.
This year, Dixon and Ramberg will pass down their directorial duties to University of Tennessee, Knoxville, physicist Sowjanya Gollapinni and Fermilab physicist Elliott McCrory. Gollapinni is a member of the MicroBooNE neutrino experiment, while McCrory, who works on Fermilab accelerators, brings with him nearly three decades of experience in training students through the Summer Internships in Science and Technology program.
While much of the physics that Dixon and Ramberg have taught has remained the same, the course has changed in other ways.
“Women joining has certainly increased over the years. It was very rare that we had more than five or six per session” in the early years, said Suzanne Weber, who has coordinated the program since 1990. Now, nearly half of each class is made up of female students.
Fermilab scientist Dan Hooper teaches the first class of Saturday Morning Physics in 2017. Photo: Elliott McCrory
Moving forward, McCrory and Gollapinni will work to continue making Saturday Morning Physics inclusive.
“We’re considering how we can add a lecture in Spanish,” McCrory said.
For Gollapinni, the importance of the lecturers as role models can’t be understated.
“It’s important for them to see all types of role models,” she said. “As a woman myself, seeing a female role model speak matters to the extent that it makes you feel like you can do anything, because you see a person like you, who has reached higher points in their career.”
Both McCrory and Gollapinni, a member of the community of university scientists who use the Fermilab for their research, also emphasized the need to experiment with the course and try new approaches to teaching physics.
“When we talk about particle physics — we’re studying essentially invisible things. So how do we know that they’re really there? It’s by the scientific method. We want the lecturers to at least touch on that concept throughout the year,” McCrory said.
Demos and other hands-on activities like tours that break up the standard two-hour lecture will also be emphasized as learning aids, Gollapinni said.
Dixon and Ramberg said they look forward to seeing where McCrory and Gollapinni take the venerated program.
“The thing that I always told the parents, which I always believed, is that the program was as much for us as it was for the kids,” Dixon said.
“This is one of the greatest tools to connect the laboratory with the community,” Ramberg said. “It really affects generations of kids.”