University of Massachusetts Amherst: A Mile Underground in the Quest for Dark Matter

Deep below the Black Hills of South Dakota, the Sanford Underground Research Facility (Sanford Lab) is hosting an innovative and uniquely sensitive dark matter detector—the LUX-ZEPLIN (LZ) experiment, led by Lawrence Berkeley National Lab (Berkeley Lab) and to which physicists at UMass Amherst contributed. The LZ experiment has passed a check-out phase of startup operations and delivered first results, the very first step in helping to determine one of the fundamental mysteries of the universe.

No one has ever seen dark matter. It does not emit, absorb or scatter light, and yet it is estimated that about 85% of the total mass of the universe comes from the substance.

Dark matter is the main source of the gravity we see in astrophysics. We need dark matter. But no one has seen a dark matter particle.

Scott Hertel, professor of physics at UMass Amherst
“There are lots of problems in astrophysics which are solved by dark matter,” says Scott Hertel, a professor of physics at UMass Amherst and whose team designed some of the techniques and equipment to calibrate the LZ detector. “There is more dark matter in the universe than normal matter. It tells us how galaxies form, how stars orbit and how things worked in the early universe, which was hot and dense. Dark matter is the main source of the gravity we see in astrophysics. We need dark matter. But no one has seen a dark matter particle. We have no idea what the particle is.”


That might soon change, thanks to the LZ experiment, which has just released results from its first 60 “live days” of testing, long enough to confirm that all aspects of the detector were functioning well. In a paper recently posted online, LZ researchers report that with the initial run, LZ is already the world’s most sensitive dark matter detector.

LZ Spokesperson Hugh Lippincott of the University of California Santa Barbara said, “We plan to collect about 20 times more data in the coming years, so we’re only getting started. There’s a lot of science to do and it’s very exciting!”

The heart of the LZ dark matter detector is buried nearly a mile underground in the Sanford Lab in Lead, S.D. The detector itself is comprised of two nested titanium tanks filled with ten metric tonnes of very pure liquid xenon and viewed by two arrays of photomultiplier tubes (PMTs) able to detect faint sources of light. LZ is designed to capture dark matter in the form of weakly interacting massive particles (WIMPs). The experiment is underground to protect it from cosmic radiation at the surface that could drown out dark matter signals.

“We’re hoping to see one atom of xenon get ‘kicked’ by one galactic WIMP particle, which will produce a brief and tiny signal: several electrons and several photons of light,” says Hertel.

Particle collisions in the xenon produce visible scintillation or flashes of light, which are recorded by the PMTs, explained Aaron Manalaysay from Berkeley Lab who, as Physics Coordinator, led the collaboration’s efforts to produce these first physics results. “The collaboration worked well together to calibrate and to understand the detector response,” Manalaysay said.

The design, manufacturing, and installation phases of the LZ detector were led by Berkeley Lab project director Gil Gilchriese in conjunction with an international team of 250 scientists and engineers from over 35 institutions from the U.S., U.K., Portugal and South Korea. The LZ Operations Manager is Berkeley Lab’s Simon Fiorucci. Together, the collaboration is hoping to use the instrument to record the first direct evidence of dark matter, the so-called missing mass of the cosmos.

Mile-deep research
Chris Nedlik spent the entirety of his UMass Amherst physics Ph.D. research working with Hertel on the LZ detector’s calibration system.

“I was a graduate student at a really fortunate time,” he says, “because I got to see it all come together, and then got to spend a few months underground in South Dakota at the end.”

Because the LZ detector is so sensitive—Hertel says that “if one innocent object that has normal background radiation, like a banana, is around the detector, it could kill the experiment”—figuring out how to calibrate it without destroying it was no small feat. So, the team build a scale model replica of the detector here at UMass and practiced calibrating it so that when they arrived in South Dakota, and confronted the real thing, they could perform their tasks flawlessly.

It was quite the experience for Nedlik. “It was a huge transition working in an underground lab,” he says. “You ride the lift down through pitch-black darkness for ten minutes and then arrive in a laboratory space that is fully finished. You wouldn’t even know you’re underground.”

“One of the reasons that I chose to get my Ph.D. at UMass,” Nedlik continues, “is that I knew the department had many opportunities for world-class research. I didn’t exactly know what I wanted to do in physics, but over the course of the time working on LZ, I was constantly in disbelief that I got to contribute to a project like this and meet such incredible colleagues.”

The take-home message from this successful startup: “We’re ready and everything’s looking good,” said Berkeley Lab Senior Physicist and past LZ Spokesperson Kevin Lesko. “It’s a complex detector with many parts to it and they are all functioning well within expectations,” he said.

Mike Headley, executive director of Sanford Lab, said, “The entire Sanford Lab team congratulates the LZ Collaboration in reaching this major milestone. The LZ team has been a wonderful partner and we’re proud to host them at Sanford Lab.” 

One of the reasons that I chose to get my Ph.D. at UMass is that I knew the department had many opportunities for world-class research. I didn’t exactly know what I wanted to do in physics, but over the course of the time working on LZ, I was constantly in disbelief that I got to contribute to a project like this and meet such incredible colleagues.

Chris Nedlik, UMass Amherst physics Ph.D. candidate
LZ is supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. LZ is also supported by the Science & Technology Facilities Council of the United Kingdom; the Portuguese Foundation for Science and Technology; and the Institute for Basic Science, Korea. Over 35 institutions of higher education and advanced research provided support to LZ. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.


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