International LZ collaboration sets a world’s best in the hunt for dark matter
Dark matter is an invisible substance that accounts for 85 percent of the mass in the universe, but determining exactly what it is remains one of the biggest questions about how the universe works. The newest results from the international LUX-ZEPLIN (LZ) collaboration, which includes Penn State researchers, extend the experiment’s search for low-mass dark matter, and setting world-leading constraints on the properties of one of the prime dark matter candidates: weakly interacting massive particles, or WIMPs. The experiment also detected signals from tiny, elusive particles called neutrinos from the sun, marking a milestone in the detector’s sensitivity and providing a window into the behavior of fundamental particles and stars.
The results were presented today in a scientific talk at the Sanford Underground Research Facility and will be released on the online repository arXiv. The paper will also be submitted to the journal Physical Review Letters.
“Although dark matter has never been directly detected, our continued searches help us set limits on the potential characteristics of WIMPs,” said Carmen Carmona, Norman and Trygve Freed Early Career Professor of Physics and leader of the LZ group at Penn State. “Even failure to detect certain events can provide important information about what these particles are, or what they aren’t, which gives us new insights into the mysterious yet ubiquitous dark matter.”
The LZ detector is managed by the 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. Researchers at Penn State made key contributions to the construction of the LZ detector, specifically on the cryogenics and liquid xenon systems, and have led several key analyses, including background modeling, detector calibrations, and dark matter sensitivity projections.
The LZ collaboration, which includes more than 250 researchers from 37 institutions worldwide, used the largest dataset ever collected by a dark matter detector, which has unmatched sensitivity. The analysis, based on 417 live days of data that were taken from March 2023 to April 2025, found no sign of lower mass WIMPs with a mass between 3 GeV/c2 (gigaelectronvolts/c2) — roughly the mass of three protons— and 9 GeV/c2. This was the first time LZ researchers have looked for WIMPs below 9 GeV/c2. The world-leading results further narrow down possibilities of what dark matter might be and how it might interact with ordinary matter.
“We have been able to further increase the incredible sensitivity of the LUX-ZEPLIN detector with this new run and extended analysis,” said Rick Gaitskell, a professor at Brown University and the spokesperson for LZ. “While we don’t see any direct evidence of dark matter events at this time, our detector continues to perform well, and we will continue to push its sensitivity to explore new models of dark matter. As with so much of science, it can take many deliberate steps before you reach a discovery, and it’s remarkable to realize how far we’ve come. Our latest detector is over 3 million times more sensitive than the ones I used when I started working in this field.”
Dark matter has never been directly detected, but its gravitational influence shapes how galaxies form and stay together; without it, the universe as we know it wouldn’t exist. Because dark matter doesn’t emit, absorb, or reflect light, researchers must find a different way to “see” it.
LZ uses 10 tonnes of ultrapure, ultracold liquid xenon. If a WIMP hits a xenon nucleus, it deposits energy, causing the xenon to recoil and emit light and electrons that the sensors record. Deep underground, the detector is shielded from cosmic rays and built from low-radioactivity materials, with multiple layers to block or tag other particle interactions – letting the rare dark matter interactions stand out.
“Reducing and modeling backgrounds is essential to LZ because any unaccounted-for signal can mimic or obscure the rare interactions expected from dark matter,” said Luiz de Viveiros, associate professor of physics at Penn State and member of the LZ experiment. “By carefully understanding and predicting all background sources, LZ can distinguish potential dark matter events from radioactive or instrumental noise, improving both its sensitivity and the reliability of its results.”
LZ’s extreme sensitivity, designed to hunt dark matter, now also allows it to detect neutrinos – fundamental, nearly massless particles that are notoriously hard to catch – in a new way. The analysis showed a new look at neutrinos from the Sun’s core, called boron-8 solar neutrinos. This provides a window into how neutrinos interact with other matter as well as the nuclear reactions in stars that produce them.
"The detection of boron-8 solar neutrinos reflects the impact of LZ’s precise calibration and deep understanding of its detector response, which results in its ability to register extraordinarily small amounts of energy from individual particle interactions,” said Carmona.
The boron-8 solar neutrinos interact in the detector through a process that was only observed for the first time in 2017, called coherent elastic neutrino-nucleus scattering or CEvNS. In this process, a neutrino interacts with an atomic nucleus as a whole, rather than just one of the particles inside it, such as a proton or neutron. Hints of boron-8 solar neutrinos interacting with xenon appeared in two detectors last year: PandaX-4T and XENONnT. Those experiments were shy of the standard threshold for a physics discovery, a confidence level known as “5 sigma,” reporting 2.64 and 2.73 sigma (respectively). The new LZ result improves the significance to 4.5 sigma, passing the 3-sigma threshold that is considered “evidence” for discoveries in science.
“Seeing these neutrino interactions was a pivotal milestone,” said Dan Kodroff, who obtained his Ph.D. at Penn State working on LZ, and is now a Chamberlain Fellow at Berkeley Lab and co-lead of the analysis. “It simultaneously showcases LZ's ability to detect signals of cosmic origin while also giving us new avenues for probing solar and neutrino physics to test the Standard Model of physics, the body of theory that describes the basic building blocks and forces of the universe.”
The neutrino signal also mimics what researchers expect to see from dark matter. That background noise, sometimes called the “neutrino fog,” could start to compete with dark matter interactions as researchers look for lower-mass particles. While the background signal from neutrinos presents challenges for the dark matter detector at low masses, its new secondary role as a solar neutrino observatory gives theorists more information for their models of neutrinos, which still hold many mysteries themselves. LZ can provide an independent measurement of how many boron-8 neutrinos are coming from the sun, known as “neutrino flux”; detect future neutrino bursts to better understand cosmic explosions called supernovae; and help study one of the fundamental parameters that describe how particles interact.
LZ is scheduled to collect over 1,000 days of live search data by 2028, more than doubling its current exposure. With that enormous and high-quality dataset, LZ will become more sensitive to dark matter at higher masses in the 100 GeV/c2 to 100 TeV/c2 (teraelectronvolt) range. Collaborators will also work to reduce the energy threshold to search for low-mass dark matter below 3 GeV/c2, and search for unexpected or “exotic” ways that dark matter might interact with xenon.
“When I step back and consider what we’ve achieved — a world-leading search for these low-mass WIMPs using the faintest signals we can see with our detector — it’s extremely rewarding, and the perfect demonstration of the experiment working as it should,” said David Woodward, who was previously a researcher at Penn State and is now the deputy operations manager for LZ at Berkeley Lab. “The result is possible because of diligent work to keep the experiment operating and collecting high-quality data over several years. It’s a team effort, with each individual bringing their care and expertise.”
Many of the researchers from LZ are also designing a future dark matter detector that uses liquid xenon on an even larger scale. The XLZD detector will combine the best technologies from projects like LZ, XENONnT, and DARWIN for a next-generation WIMP hunter that can also study neutrinos, the sun, cosmic rays, and other unusual candidates for dark matter, such as dark photons and axion-like particles.
LZ is supported by the U.S. Department of Energy, Office of Science, Office of High Energy 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-seven institutions of higher education and advanced research provided support to LZ. The LZ collaboration acknowledges the assistance of the Sanford Underground Research Facility.
Editor's Note: A version of this release originally appeared on the Berkeley Lab website.