The international research team has been unable to explain the interaction with known background signals from normal matter – although the result does not yet meet the statistical threshold required to claim a discovery.
Dark matter 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 in science.
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 weakly interacting massive particles, WIMPs.
In the new result, researchers analysed 220 live days of data collected between March 2023 and April 2024. Professor Chamkaur Ghag, of UCL’s Department of Physics and Astronomy, served as LZ’s international spokesperson through this period, leading the collaboration’s global effort. The UK plays a leading role in the analysis behind this result, and UCL researchers have supported much of that work, from understanding and modelling detector backgrounds to developing the statistical inference tools used in the analysis.
The results were presented in a scientific talk at the 2026 TeV Particle Astrophysics conference in Japan. The paper will be released on the online repository arXiv and submitted to the journal Physical Review Letters.
Professor Ghag said: “This single event sits exactly where we would hope dark matter to appear, in a part of our search we had not fully explored before, and it has withstood months of scrutiny. We are not claiming a discovery, but this is a genuinely exciting moment.
“We built LZ to be sensitive enough, and clean enough, to catch something like this if it were there. Whether this event turns out to be dark matter or an extraordinarily unlucky background, the coming years of data will tell us, and either way it’s a strong test of everything the collaboration has built.”
The collaboration had previously searched this dataset for faint signals from the simplest kinds of WIMP interactions, a search that delivered the world’s most stringent constraints to date and was led by Dr Amy Cottle, of UCL’s Department of Physics and Astronomy. The new analysis searched for a broader range of possible WIMP interactions that could deposit more energy in the detector.
Dr Cottle said: “The traditional WIMP mass range is where most of the field has focused for decades, so setting the world-leading limit there was already a significant result, and a big moment for the UK’s contribution to LZ and the field. But a lot of the value in a search like that comes from knowing your detector so well that you can say with confidence when something doesn’t fit. That’s what years of work gave us and helped this event stand out when we went looking somewhere new.
Lead author Sam Eriksen, based at the University of Bristol, said: “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. 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.”.
If the anomalous event was caused by dark matter, the WIMP that generated it would likely have a mass of at least 200 GeV/c² (gigaelectronvolts), or more than 200 times the mass of a proton. It would also suggest a specific type of interaction between WIMPs and ordinary matter beyond the simplest model. The LZ results have not reached “5-sigma” significance, the statistical threshold considered a discovery in physics. The new analysis is 2.6 sigma, meaning there is approximately a 0.5% chance that the event could be explained by known backgrounds.
Professor Rick Gaitskell, the current spokesperson for LZ based at Brown University, emphasised caution around over-interpreting the result. He said: “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. 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.”
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.
Dr Aaron Manalaysay, physicist at Berkeley Lab and chair of LZ’s Institutional Board, said: “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. 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.”
With additional data, researchers can test whether the finding continues to grow in significance or fades away. LZ has already accumulated the world’s largest dark matter dataset and will continue to accrue WIMP search data at SURF, substantially improving their search statistics.
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; the UK’s Science & Technology Facilities Council; 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. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.