Did the LUX-ZEPLIN Experiment Detect a Dark Matter Particle?
A single particle interaction could be a signal from a dark matter candidate, though further study is needed.
The LUX-ZEPLIN main detector in a surface lab before installation underground. (Credit: Matthew Kapust/Sanford Underground Research Facility)
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 science’s biggest questions.
Now, a new analysis from the LUX-ZEPLIN (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, including The University of Texas at Austin. 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 tons of ultrapure liquid xenon to search for dark matter and is optimized to look for WIMPs, or weakly interacting massive particles.
The results, made possible in part by funding from the U.S. Department of Energy, 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.
“What makes this so exhilarating is that even after many months of intense review by all our collaborators, we have yet to come up with a plausible alternative explanation beyond some new interaction never before observed,” said Scott Kravitz, assistant professor of physics at UT Austin and LZ analysis coordinator during most of this search. “We don’t want to overstate this result— with just one event, we’re not claiming a definitive dark matter discovery. But it’s hard not to get excited about that possibility as LZ takes and analyzes more data. It’s a good time to be looking for dark matter!”
To search for dark matter, LZ uses photomultiplier tubes (shown here before installation in the detector) to capture light from particle interactions. (Credit: Matthew Kapust/Sanford Underground Research Laboratory)
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.
“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.”
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.
LZ uses a cylindrical chamber full of liquid xenon to search for dark matter. It is surrounded by additional layers to detect or block background particles (left). When a WIMP collides with a xenon atom (right), it 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)
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.
“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.”
LZ is supported in part by the U.S. Department of Energy (DOE), 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. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.
Adapted from a press release by Lawrence Berkeley National Laboratory.
