The LZ central detector in the clean room at Sanford Lab before beginning its journey underground. Photo by Matthew Kapust, Sanford Underground Research Facility.
The findings, while incremental, represent a significant advance, Gaitskell added. They help researchers narrow their search, improving their understanding of dark matter, if it exists. For instance, the experiment’s ability to detect extremely faint interactions now allows researchers to rule out many possible dark matter models that predict interactions stronger than what the data shows, ultimately dwindling down the options for where WIMPs could be hiding.
“Our new 8-tonne LZ detector is more massive and more sensitive than any previous direct search experiment,” Gaitskell said. “We are potentially able to see things that no one has seen before.”
Zeroing in on dark matter
The LZ detector is located a mile below the Black Hills of South Dakota in the Sanford Underground Research Facility. The descent into the former gold mine, which was also home to the precursor LUX experiment until 2016, takes about 10 minutes via elevator. The heart of the LZ consists of two nested titanium tanks filled with about 10 tons of pure liquid xenon. The tanks are monitored by photomultiplier tubes (PMTs) that will detect the dark matter particles if they’re there.
The theory for this detection works like this: If WIMPs are present, they may occasionally collide with the nucleus of a xenon atom, causing a tiny flash of light and some movement in the atoms. If that happens, the PMTs are meant to catch that interaction. Capturing that glimpse, of course, is exceedingly difficult and why most researchers instead infer things about dark matter through how its powerful gravity bends and focuses the light around it, a phenomenon called gravitational lensing.
Much work done on the two massive photodetector arrays happened in Barus and Holley, including building, testing and integrating more than 14,000 components. Photo by Nick Dentamaro.
In 2022, the detector passed a check-out phase of startup operations and delivered its first results, proving itself to be world’s most sensitive detector of dark matter and placing what were until now the strictest limits on how strongly WIMPs should interact with ordinary matter. To get the latest result, the analysis combined 220 days of new data taken in 2023 and 2024 with 60 days from the experiment's first run, which started in 2022. LZ is still in its early phases, too. By 2028, the LZ team plans to gather over 1,000 days of data.
“This is how science works,” Gaitskell said. “This is a huge question we are trying to answer: ‘What is most of the matter in the universe made of?’ It won’t get answered in a matter of weeks, months or even years necessarily. This is one that will take decades to answer, and that is actually pretty typical of most major scientific questions. If you go back and look at the history books, you'll realize that major discoveries are separated by long periods of time and happened step-by-step.”
This new results marks the first time that LZ has applied “salting”— a technique that adds fake WIMP signals during data collection. By camouflaging the real data until “unsalting” at the very end, researchers can avoid unconscious bias and keep from overly interpreting or changing their analysis before knowing the outcome.
“There’s a human tendency to want to see patterns in data, so it’s really important when you enter this new regime that no bias wanders in,” said Scott Haselschwardt, the LZ physics coordinator and an assistant professor at the University of Michigan. “If you make a discovery, you want to get it right.”
Brown’s role in the experiment
Since its start, early-career researchers have played a significant role in building and operating the experiment. Brown’s team, for instance, was crucial in building the PMT arrays, which serve as the experiment’s “eyes.”
The group worked with Berkeley Lab and Imperial College London researchers to design, test and assemble the arrays and its more than 14,000 components in cleanrooms at Brown’s Department of Physics, where the components also underwent two years of testing.
A photomultiplier tube is carefully inserted into the array inside the PMT Array Lifting And Commissioning Enclosure. Photo by Nick Dentamaro.
“The work didn’t stop once we started operating, either,” Gaitskell said. “Brown researchers have been directly responsible for continuing to run the systems that we built and oversee its care.”
This often involves hands-on work at the site itself. Brown physics Ph.D. student Chen Ding, for instance, spent three weeks in South Dakota performing crucial calibration tasks last summer. The experience was surreal, he said.
“Once you enter the lab, it's like a normal lab, but it's a very special experience getting there,” Ding said. “It's not just going down the elevator for 10 minutes — you still need to walk what feels like a mile down there and take a little train, all while there’s other excavation going on, and you’re in full safety gear.”
Another Brown Ph.D. student, Austin Vaitkus, who monitors the PMTs' performance, said the experience is invaluable to researchers, like him, at the start of their career. “When you are working on an experiment like this, you are working with the best people in the world on this subject,” Vaitkus said. “It’s really exciting to be able to say that you’re on a team that’s the best at doing what they do.”
Brown doctoral candidate Jihyeun Bang shares that enthusiasm, emphasizing the broader impact of the work: “Dark matter is one of the biggest challenges in the field, and it’s been motivating to see my work directly contribute to that overall goal.”
LZ is supported by the U.S. Department of Energy, the Science and Technology Facilities Council of the U.K., the Portuguese Foundation for Science and Technology, the Swiss National Science Foundation, and the Institute for Basic Science in Korea, with assistance from the Sanford Underground Research Facility.
