Intriguing result in dark matter search

The LUX-ZEPLIN (LZ) experiment observed a particle interaction that could be interpreted as a signal from weakly interacting massive particle, or 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.

‘The mystery of dark matter is one so central to our understanding of the universe that it has motivated decades of technological and analytic development, as well as the passionate investment of the time and effort of researchers like me and our wider LZ team,’ remarked Professor Kimberly Palladino from the Department of Physics at the University of Oxford.

Now, a new analysis from the LZ experiment has recorded a single particle interaction 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.

LZ is an international collaboration of 250 scientists and engineers from 39 institutions. The detector is managed by the US 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 optimised to look for WIMPs.

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.

‘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 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 analysed 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 minimising false positives.

‘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 UK 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.’

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.

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.

‘This is an exciting result which needs to be treated with care as we endeavour to uncover the nature of this event,’ comments Oxford DPhil student Nick Fieldhouse. ‘Going forward the simulations developed here in Oxford will play a crucial role in verifying the various signal models we wish to investigate.’

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.

‘As the song says, one is the loneliest number,’ said Professor Palladino, continuing, ‘but to understand this event as a novel background or a signal we will need to analyse more data. And these searches don't just end with LZ, but our planned future detector XLZD continues this technology to search for heavy particle dark matter, as well as the search for neutrino-less double beta decay. A large liquid xenon detector is truly a rare event observatory, and the UK has the opportunity to host this major international facility.’ XLZD brings together experimentalists from LZ, the competing XENONnT experiment that announced the low energy detection of solar neutrinos on Monday, as well as members of nEXO and other low background experiments.

LZ is supported by the US 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 and 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. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.