Research

UMass Amherst Physicists on International Team Make What May Be The First Observation of Dark Matter

The LUX-ZEPLIN experiment observed a particle interaction that could be interpreted as a signal from WIMPs, a dark matter candidate — but researchers will need more data to confirm

For the better part of a century, people have been trying to understand dark matter. This invisible substance makes up roughly 85% of the mass in the universe but has never been directly detected. Determining exactly what it is remains one of the biggest questions about our world.

Now, a new analysis from the LUX-ZEPLIN (LZ) experiment has recorded a single flash of light one mile beneath the Earth’s surface that researchers have great difficulty explaining with known background signals from normal matter. The result does not yet meet the statistical threshold required to claim a discovery, but is the most compelling hint of dark matter reported by the experiment to date.

“We’re working very hard to keep the meaning of this one event in perspective,” says Scott Hertel, associate professor of physics at the University of Massachusetts Amherst, who, along with his graduate student, Mark Murdy, is among the 250 scientists across the globe who have contributed to the LZ experiment. “We saw something, a nuclear kick at a particular energy and in a particular location within the detector. And this recoiling nucleus can’t be easily explained by any of the physics we now have. We’ve gone through the list of possible explanations, and none of them fit—but we could have missed something. This is not yet a discovery, but it is how a discovery might start.”

LZ is an international collaboration of 250 scientists and engineers from 39 institutions. The detector is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and operates nearly one mile below ground at the Sanford Underground Research Facility (SURF) in South Dakota. The experiment uses 10 tonnes of ultrapure liquid xenon to search for dark matter and is optimized to look for WIMPs, or weakly interacting massive particles.

The results were presented this week in a scientific talk at the 2026 TeV Particle Astrophysics conference in Japan. The paper has been released in the online repository arXiv and will be submitted to the journal Physical Review Letters for peer review.

When a WIMP collides with a xenon atom, the xenon atom emits a flash of light and electrons. The light is detected at the top and bottom of the liquid xenon chamber. An electric field drifts the electrons to the top of the chamber, where they generate a second flash of light. Credit: Greg Stewart, SLAC National Accelerator Laboratory

“We’re very intrigued to see this event in the data, in the region where we expect dark matter to show up and the competing backgrounds are very low,” said Rick Gaitskell, a professor at Brown University and the spokesperson for LZ. “With only one event, we don’t want to get ahead of ourselves. We are not claiming to have seen dark matter. But we have seen something interesting that we want to share with the scientific community for their input.”

The LZ collaboration studies experimental data in batches. In the new result, researchers analyzed 220 live days of data collected between March 2023 and April 2024. The collaboration had previously searched this dataset for faint signals from the simplest kinds of WIMP interactions. The new analysis searched for a broader range of possible WIMP interactions that could deposit more energy in the detector. LZ is particularly sensitive to such signals while also minimizing false positives.

Physics graduate student, Mark Murdy, a mile underground at the LZ site. Credit: Scott Hertel

“This was a detailed study in a region we hadn’t explored within this dataset, and we spent months of additional effort to understand all the possible causes of background events,” said Sam Eriksen, a senior research associate at the University of Bristol in the U.K. and lead author of the study. “We understand our detector and the backgrounds so well that even a single outstanding event, like the one we found, is important. We expect dark matter events to be extremely rare, so only a handful could mark the first detection of WIMP dark matter.”

“We have huge amounts of new data to analyze, and more to come as the experiment continues to run” says Hertel. “When we’re done, we’ll be able to say something much more definitive about WIMPs and dark matter.”

“This is the first example in any experiment I've worked on of an outlier that appears valid in every way,” says Aaron Manalaysay, a physicist at Berkeley Lab and the chair of LZ’s Institutional Board. “Of course, we’re still twisting our brains trying to think if there’s a rare background mechanism we could’ve missed, but it’s thrilling to wonder if this could be the first hint of a dark-matter observation.”