LZ sees surprising result in search for dark matter

Key takeaways:

• Dark matter is the mysterious substance that accounts for the vast majority of matter in our universe. 
• LUX-ZEPLIN (LZ) is a world-leading dark matter detector and specializes in searching for WIMPs, or weakly interacting massive particles.
• A new LZ study found one particle interaction that is difficult to explain with known background processes and could potentially have been caused by a WIMP.  

• LZ continues to run to see if additional data strengthens or weakens the hint of a dark matter interaction.

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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 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 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. 

Researchers at the Department of Energy's SLAC National Accelerator Laboratory have helped shape LZ since the experiment was first proposed. 

“It’s exciting to look back over the last dozen years since Tom Shutt and I started the LZ group at SLAC,” said Dan Akerib, a professor of particle physics and astrophysics at SLAC and Stanford University. “Without the lab’s support and the technical talent here it would have been impossible for us to have built such an effective team, which has had a huge impact on the experiment.”

In addition to developing the systems that purify the liquid xenon to the extraordinary levels required for LZ’s dark matter search and the state-of-the-art custom electrodes that collect and develop the signals in the liquid xenon, SLAC researchers contributed to detector assembly and integration hardware, background modeling, data analysis and offline data reduction computing systems that laid the foundation for the experiment.

“This experiment relies on having a detector with extraordinarily low levels of trace radioactivity, and also the sophistication to distinguish between background signals caused by trace radioactivity and signals from dark matter,” said Tom Shutt, a professor of particle physics and astrophysics at SLAC and Stanford University. “Achieving that has been a real testament to the collaboration.”

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 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.

“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 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.

SLAC researchers contribute to LZ's day-to-day operations and the ongoing analysis of its growing datasets, developing techniques to distinguish potential dark matter interactions from background signals. 

“The team at SLAC has made vital contributions to the data analysis and physics results since LZ turned on,” said Ann Wang, a Panofsky Fellow at SLAC who serves as co-deputy physics coordinator for LZ. “The students, postdoctoral researchers and staff across the years have driven improvements across almost all aspects of LZ data analysis, from the event reconstruction to the background modeling.”

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 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. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.

Adapted from a release by Lawrence Berkeley National Laboratory. Read the original here.

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