The LUX-ZEPLIN, or LZ, experiment has reported a compelling result in its search for dark matter. Although the result doesn’t meet the statistical threshold for claiming a discovery of dark matter, LZ’s international research team, including members from the University of Michigan, cannot currently explain it with known background signals.

“It’s certainly not evidence that we’ve seen dark matter, and maybe it’s too much to say that it’s even a hint, but it’s interesting enough that we’re going to pursue it with our full strength,” said Wolfgang Lorenzon, professor of physics and principal investigator of the U-M dark matter group, who has been working on the LZ experiment since 2015.

Wolfgang Lorenzon

The LZ collaboration includes 250 scientists and engineers from 39 institutions. The detector is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and operates nearly one mile below ground at the Sanford Underground Research Facility, or SURF, in South Dakota. The experiment uses 10 metric tons of ultrapure liquid xenon to search for dark matter and is optimized to look for weakly interacting massive particles, or WIMPs.

The new 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.

Rick Gaitskell

“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.”

When a WIMP collides with a xenon atom, the xenon atom 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.
Scott Haselschwardt

Why a WIMP would be mighty exciting

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 in physics.

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.

U-M researchers, including Lorenzon and Scott Haselschwardt, have played key roles in developing systems that minimize those mimics and exclude ones that make it through. Members including U-M graduate student Kaiyuan “Sky” Shi and former U-M postdoc Gregory Rischbieter have been integral in LZ’s data analysis efforts.

In particular, the U-M team led the development and validation of models and calibration studies that extended LZ’s sensitivity beyond the traditional WIMP search region. This directly enabled the study to test a broader range of possible interactions, Shi said.

The LZ collaboration studies experimental data in batches, looking for signals of dark matter candidates. 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.

Shi was heavily involved in the analysis that identified the event, as well as ruling out different backgrounds or physical processes that could account for its origin. When he first saw it, he knew it was something special.

“That was the happiest we’ve been all year. At first, people didn’t know what to do,” Shi said. “But then we started saying, ‘OK, if this is a real event, we need to be more rigorous. We need to ask more questions.’ And every time we solved one question, we came up with two more.”

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 known backgrounds could explain the event.

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 with more than 700 live days of data and will continue to accrue WIMP search data at SURF, substantially improving its search statistics.

“Now is a really interesting time in the search for dark matter,” said Haselschwardt, an assistant professor of physics who started working on LZ when he was a graduate student. “We have data already collected that will help us understand what this event might be and whether it’s consistent with a dark matter signal. I’m very excited for the future—even the next year or two are going to be really important.”

With that excitement, though, comes a healthy amount of skepticism the researchers said. In fact, part of the reason the team is publishing this result now is to put it out to the dark matter community and beyond to think about other explanations.

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; Portuguese Foundation for Science and Technology; the Swiss National Science Foundation; Australian Research Council Centre of Excellence for Dark Matter Particle Physics; and 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.

Written by Lauren Biron

More Information:
Wolfgang Lorenzon, U-M professor of physics
Rick Gaitskell, Brown University professor and LZ spokesperson
Scott Haselschwardt, U-M professor of physics
Kaiyuan “Sky” Shi, U-M graduate student
Gregory Rischbieter, former U-M postdoc

Study: Search for dark matter particle interactions in an extended nuclear recoil energy window with the LUX-ZEPLIN (LZ) experiment