A record-breaking investigation, using a particle detector a mile underground in South Dakota, may have revealed new insights about dark matter, the mysterious substance believed to make up most of the matter in the universe.
Using the largest dataset of its kind, the experiment — called LUX-ZEPLIN (LZ) — constrained the potential properties of one of the leading candidates for dark matter with unprecedented sensitivity. The research did not uncover any evidence of the mysterious substance, but will help future studies avoid false detections and better hone in on this poorly understood piece of the universe.
WIMPs vs. neutrinos
The team had two goals for the new study: to elucidate the properties of a low-mass “flavor” of proposed dark-matter particles called weakly interacting massive particles (WIMPs), and to see if the detector could view solar neutrinos — nearly mass-less subatomic particles produced by nuclear reactions inside the sun. The team suspected that the detection signature of these particles could be similar to that predicted by certain models of dark matter, but needed to spot the solar neutrinos to know for sure.
Before the experiment, which took 417 days to perform between March 2023 and April 2025, the detector’s sensitivity was upgraded to search for rare interactions with fundamental particles. A cylindrical chamber filled with liquid xenon was the theater for action. Researchers could watch for either WIMPs or neutrinos colliding with the xenon, either of which produces flashes of photons, along with positively charged electrons.
The experiment pushed forward the science for both the WIMP and neutrino questions. For the neutrinos, researchers improved their confidence that a type of solar neutrino, known as boron-8, is actually interacting with the xenon. This knowledge will help future studies avoid false detections of dark matter.
Physics discoveries typically must reach a confidence level called “5 sigma” to be considered valid. The new work achieved 4.5 sigma — a considerable improvement over sub-3-sigma results reported in two detectors last year. And that was especially notable given that boron-8 detections happen only about once a month in the detector, even when monitoring 10 tons of xenon, Gaitskell said.
As for the dark matter question, however, the researchers didn’t find anything definitive for the low-mass types of WIMPs they were seeking. Scientists would have known it if they saw it, the team said; if a WIMP hits the heart of a xenon molecule, the energy of the collision creates a distinctive signature, as best as models predict.
“If you take a nucleus, it is possible for dark matter to come in and actually simultaneously scatter from the entire nucleus and cause it to recoil,” Gaitskell explained. “It’s known as a coherent scatter. It has a particular signature in the xenon. So it’s those coherent, nuclear recoils that we’re looking for.”
The team did not detect this signature in their experiment.
Doubling the run
Another, longer run will begin in 2028, when the detector is expected to collect results for a record-breaking 1,000 days. Longer runs give researchers a better chance of catching rare events.
The detector will hunt not only for more solar neutrino or WIMP interactions but also other physics that may fall outside the Standard Model of particle physics said to describe most of the environment around us.
Gaitskell emphasized that the role of science is to keep pushing forward even when “negative” results arise.
“One thing I’ve learned is, don’t ever assume that nature does things in the way that you think it should, exactly,” said Gaitskell, who has been studying dark matter for more than four decades.
“There are plenty of elegant [solutions] that you would say, ‘That’s so beautiful. It has to be true.’ And we tested them … and it turned out, nature ignored it and nature did not want to go down that particular route.”