Physics undergraduate student Minjie Lei performs a measurement with the prototype Radon Removal system for the LZ gas circulation element at the University of Michigan. (Credit Kirill Pushkin/U-M)

ANN ARBOR--A team at the University of Michigan Department of Physics is part of the race to build the most sensitive U.S.-based experiment designed to directly detect dark matter particles. Department of Energy officials formally approved a key construction milestone, known as Critical Decision 3 (CD-3), recently 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. 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-generation dark matter direct-detection experiments planned in Italy and China. The LZ collaboration now has about 220 participating scientists and engineers who represent 38 institutions around the globe. The LZ team of physicists at the University of Michigan includes, Professor Carl Akerlof, Professor Wolfgang Lorenzon, Research Scientist Michael Schubnell, Postdoctoral Fellow Kirill Pushkin, Graduate Student Maris Arthurs and Undergraduate Students Dhayaa Anbajagane, Minjie Lei, Michael Reh, Divyanish Saini, John Schaefer and Yuhan Wang.

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.

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 the 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.”

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.

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. The U-M team is leading the effort to purify xenon for LZ by removing radon that is constantly emanating from detector components that are in direct contact with xenon.

“We have completed the prototype system and are now embarking on the final radon removal system that will increase the performance, and thus the science reach of LZ”, said Wolfgang Lorenzon, the PI of the U-M team. 

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: 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.