Researchers with the LUX-ZEPLIN dark matter experiment recorded a mysterious single particle interaction during observations gathered between March 2023 and April 2024. Operating nearly a mile underground in South Dakota, the international team reported a global statistical significance of 2.6 sigma after accounting for the “look-elsewhere” effect — well below the level required to claim a discovery.
Inside the Underground Detector Searching for WIMPs
For nearly a century, scientists have been trying to identify dark matter, the invisible material thought to account for about 85% of all matter in the universe. Its gravitational effects can be seen throughout the cosmos, yet no experiment has directly detected the substance itself, leaving discovering what dark matter is made of as one of the most important unresolved problems in modern physics. Deep beneath the surface at the Sanford Underground Research Facility in South Dakota, an international project involving 250 scientists and engineers from 39 institutions is probing this question. The experiment is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).
To capture direct evidence, the LUX-ZEPLIN (LZ) experiment deploys a massive instrument using about 10 tonnes of ultrapure liquid xenon. The instrument was designed primarily to search for WIMPs, or weakly interacting massive particles, one of the leading theoretical candidates proposed to explain dark matter. LZ searches for dark matter by looking for signature flashes of light from energy deposited in the detector. The collaboration leverages multiple methods to prevent or account for particle interactions caused by normal matter. This includes the mile of rock that shields the detector from cosmic rays from space, a water tank and outer detectors that protect the central detector from background neutrons, and a suite of computational tools that disentangle particle interactions and reject dark matter mimics.
Uncovering an Unexplained Outlier in the Data
The new results were presented during a scientific talk at the 2026 TeV Particle Astrophysics conference in Japan, and the paper will also be posted to arXiv and submitted to Physical Review Letters. The LZ collaboration examines its experimental results in batches. For this latest study, scientists analyzed 220 live days of observations gathered between March 2023 and April 2024. Researchers had previously searched the same dataset for very faint signatures associated with the simplest forms of WIMP interactions. This time, they expanded the search to include a wider variety of possible WIMP interactions capable of depositing larger amounts of energy inside the detector. LZ is especially sensitive to events of this kind, while its design also helps scientists reduce the chances of mistaking ordinary particle interactions for dark matter.
That expanded search uncovered a particularly intriguing event: researchers recorded a single particle interaction that has so far proved difficult to explain using known background signals produced by ordinary matter. 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.”
The research team spent months vetting the unexpected signal. Outlier events in the data are not unexpected, but they usually stand out as a background of some kind when you look at them deeper,
said Aaron Manalaysay, a physicist at Berkeley Lab and the chair of LZ’s Institutional Board. “This is the first example in any experiment I’ve worked on of an outlier that appears valid in every way. 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.”
Weighing the Odds Against a Formal Discovery
The finding is not statistically strong enough to qualify as a discovery, representing the most compelling potential dark matter signal LZ has reported so far. LZ is supported by the U.S. Department of Energy, Office of Science, Office of High Energy and Nuclear 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; the Swiss National Science Foundation; the Australian Research Council Centre of Excellence for Dark Matter Particle Physics; and the Institute for Basic Science, Korea. Thirty-nine institutions of higher education and advanced research provided support to LZ, and the collaboration acknowledges the assistance of the Sanford Underground Research Facility.
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.
What Comes Next for the LZ Collaboration
Researchers will continue collecting data to determine whether the signal strengthens or disappears as more observations are made. Although researchers stress that the finding does not amount to a discovery, the recorded particle interaction could be one of the strongest clues yet to the nature of the invisible material.
