PROVIDENCE, R.I. [Brown University] — For the better part of a century, scientists 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 unanswered questions about the universe.
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 it is the most compelling hint of dark matter reported by the experiment to date.
“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 of physics 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 results were presented in a scientific talk at the 2026 TeV Particle Astrophysics conference in Japan on Tuesday, Sept. 1. A paper detailing the finding will be released on the online repository arXiv and submitted to the journal Physical Review Letters. The paper and talk can be downloaded via the LZ Experiment website. Gaitskell will give a presentation on the findings at a Physics Department colloquium on Sept. 11.
While dark matter has never been detected directly, scientists have seen evidence of its existence through its gravity. The extent to which light bends as it passes by large galaxies suggests there’s more mass in those galaxies than can be accounted for by normal matter (like stars and planets) alone. That extra mass, scientists surmise, could be particles that neither emit nor reflect light, and interact only very rarely with other forms of matter. A leading theoretical candidate for a dark matter particle is the weakly interacting massive particle or WIMP.
The LZ detector, which is optimized to search for WIMPs, is a 10-ton tank of ultrapure liquid xenon equipped with sensors designed to record rare interactions between a WIMP and the xenon atoms in the tank. Managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), the detector operates nearly one mile below ground at the Sanford Underground Research Facility (SURF) in South Dakota, where it is shielded from cosmic radiation and other types of background noise that could mimic a dark matter interaction.
The LZ experiment is an international collaboration of 250 scientists and engineers from 39 institutions.
Gaitskell, along with dozens of Brown graduate students and postdoctoral researchers, have been involved in the dark matter search for decades, including work with LZ and its predecessor, the LUX experiment. Brown physics Ph.D. students Benjamin Almquist, Chen Ding and Chongwen Lu played key roles in analyzing the latest round of new data and in overseeing the detector’s day-to-day operations. Undergraduate Woody Hulse, graduate student Charles Kong and Shawn Dubey, a postdoctoral fellow at the Brown Center for the Fundamental Physics of the Universe, developed new machine learning algorithms for LZ data analysis.
Brown researchers also helped to build the detector itself, including construction of the powerful light sensors used to look for the signature flashes of light associated with WIMP interactions inside the detector. LZ has been in operation since 2021, when it began its record-breaking search for dark matter particles.
New results
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. The LZ results have not reached “5-sigma” significance, the statistical threshold in physics that an event must reach to be considered a discovery. 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.
JiJi Fan, an associate professor of physics at Brown not involved in the LZ experimental research but whose theoretical work is cited in the new LZ paper, said the new findings were “very intriguing.” While the event recorded by LZ could not have been the simplest type of WIMP interaction, there are other types of possible interactions that could explain it, she said. Had a WIMP interacted in the simplest way at the high energy associated with this recent event, the detector would have recorded many recoil events at lower energies following the initial event. LZ recorded a single event, however, which requires an expanded theoretical explanation.
“There are well-motivated theoretical extensions, including inelastic scattering and momentum-dependent elastic scattering, that could potentially account for this single event,” Fan said. “This also sets up a target for on-going dark matter hunting, not only at direct detection experiments such as LZ but also at other complementary experimental frontiers.”
