Construction Moves Forward on Next-Gen Dark Matter Detector

The next-generation dark matter detector LUX-ZEPLIN (LZ) has cleared another approval milestone and is on schedule to begin its deep-underground hunt for theoretical particles known as WIMPs, or weakly interacting massive particles, in 2020. Physicists consider WIMPs among the top prospects for explaining dark matter, the unseen stuff that scientists have observed only through gravitational effects, and LZ, with its 100 times increase in sensitivity, is thought to be the next best chance to detect dark matter interactions with matter.

“The nature of the dark matter, which comprises 85 percent of all matter in the universe, is one of the most perplexing mysteries in all of contemporary science,” says Harry Nelson, LZ spokesperson and a physics professor at University of California, Santa Barbara. “Just as science has elucidated the nature of familiar matter—from the periodic table of elements to subatomic particles, including the recently discovered Higgs boson—the LZ project will lead science in testing one of the most attractive hypotheses for the nature of the dark matter.”

LZ is named for the merger of two dark matter detection experiments: the Large Underground Xenon experiment (LUX) and the UK-based ZonEd Proportional scintillation in Liquid Noble gases experiment (ZEPLIN). The project is supported by a collaboration of more than 30 institutions and about 200 scientists worldwide.

Last month, LZ received an important US Department of Energy approval (known as Critical Decision 2 and 3b) for the project’s overall scope, cost and schedule. The latest approval sets in motion the buildout of major components and the preparation of its nearly mile-deep lair at the Sanford Underground Research Facility (SURF) in Lead SD.

“Nobody looking for dark matter interactions with matter has so far convincingly seen anything, anywhere, which makes LZ more important than ever,” says Murdock Gilchriese, LZ project director and Berkeley Lab physicist.

Engineers designed LZ to tease out dark matter signals from within a chamber filled with 10 metric tons of purified liquid xenon, one of the rarest elements on Earth, and a “nearly magical substance for WIMP detection,” according to Henrique Araujo, a professor from Imperial College London who leads the project in the UK.

Liquid xenon was selected because it can be ultra-purified, including the removal of most traces of radioactivity that could interfere with particle signals, and because it produces light and electrical pulses when it interacts with particles. Xenon gas, which is costly to produce, is used in lighting, medical imaging and anesthesia, space-vehicle propulsion systems and the electronics industry. The entire supply of xenon for the project is already under contract, purchased in part by the state of South Dakota.

Experiments seeking traces of dark matter have grown increasingly sensitive in a short time, according to Gilchriese. “It’s really like Moore’s law,” he says, referencing an observation about regular, exponential growth in computing power through the increasing concentration of transistors on a computer chip over time. “The technologies used in liquid xenon detectors have been demonstrated around the world.”

Engineers at SLAC National Accelerator Laboratory will purify the xenon before it’s delivered in gas form in tanks to LZ. “Having focused on design and prototyping for some time now, it’s very exciting to be moving forward toward building the LZ detector and the production-scale purification systems that will process its xenon,” says Dan Akerib, co-lead on SLAC’s LZ team. “The goal is to limit contamination from another element, krypton, to just one-tenth of a part per trillion.”

Teams at Fermi National Accelerator Laboratory (CSA CSM) and the University of Wisconsin’s Physical Sciences Laboratory are also contributing to LZ, working together to ensure that none of that expensive xenon is lost should there be a power outage or extended down time.

“The xenon in LZ is precious both scientifically and financially, so it’s very important that we have the same amount of xenon at the end of the experiment as at the beginning,” says Hugh Lippincott of Fermilab, the current physics coordinator of the collaboration. “We’re excited to be part of this next generation of direct dark matter experiments.”

LZ is designed so that a dark matter particle 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, will carry the telltale fingerprint of the particles that created them.

An undisclosed company in Japan is manufacturing the tubes, while another manufacturer in Italy is constructing ultrapure titanium sheets that will be formed, fitted and welded together to create a double-walled vessel to hold the liquid xenon.

In recent weeks, researchers used the soon to be dismantled LUX experiment as a test bed for prototype LZ electronics, examining new approaches to monitoring and measuring particle signals that will help fine-tune the LZ detector.

Other work is focused on precisely measuring the slightest contribution to background noise in the detector posed by all of the components that will surround the liquid xenon, to help predict what the detector will see once it’s turned on. Researchers at Berkeley Lab are testing a high-voltage system that will generate an electric field within the detector to guide the flow of electrons produced in particle interactions to the top of the liquid xenon chamber.

In the next year there will be lot of work at the Sanford Underground Research Facility in South Dakota to disassemble LUX and prepare the underground site for planned LZ assembly and installation in 2018 and 2019.

“We have learned a ton of stuff from LUX,” McKinsey says. “We are mixing in some different forms of elements that we can remove really well or that decay to stable isotopes—to measure all of the responses of the liquid xenon detector. We are making sure our errors are small when we actually do the LZ experiment.”