LZ experiment hints at dark matter

The LUX-ZEPLIN (LZ) experiment has recorded a single, unexplained particle interaction in its search for dark matter. The result, presented at the 2026 TeV Particle Astrophysics conference in Japan, is the most compelling hint of dark matter the experiment has reported, though it does not yet meet the statistical threshold for a discovery.

LZ is an international collaboration of 250 scientists and engineers from 39 institutions. The detector, managed by the U.S. Department of Energy's Lawrence Berkeley National Laboratory, operates nearly one mile underground at the Sanford Underground Research Facility in South Dakota. It uses 10 tonnes of ultrapure liquid xenon to search for weakly interacting massive particles (WIMPs), a leading dark matter candidate.

An intriguing outlier event

Researchers analyzed 220 live days of data collected between March 2023 and April 2024. The new analysis searched for a broader range of possible WIMP interactions than previous studies of the same dataset. In this search, one event stood out.

"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. He emphasized that with only one event, the collaboration is not claiming a discovery. Sam Eriksen, a senior research associate at the University of Bristol and lead author of the study, noted the team spent months understanding all possible background causes. "Our knowledge of the detector and the backgrounds is so thorough that even a single outstanding event, like the one we found, is important," Eriksen said.

Interpreting the signal

If the event was caused by dark matter, the WIMP responsible would likely have a mass of at least 200 GeV/c², 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 theoretical model.

The statistical significance of the new analysis is 2.6 sigma. This means there is approximately a 0.5% chance the event could be explained by known backgrounds. This falls short of the 5-sigma threshold considered a discovery in particle physics.

Aaron Manalaysay, a physicist at Berkeley Lab and chair of LZ's Institutional Board, called the event unique. "This is the first example in any experiment I've worked on of an outlier that appears valid in every way," he said. The team continues to investigate whether a rare background mechanism could have been missed.

The path forward with more data

LZ will continue to collect data. The experiment has already accumulated the world's largest dark matter dataset. Additional data will determine if the significance of this finding grows or fades away.

The detector searches for signature flashes of light from energy deposited in the liquid xenon. It is shielded by a mile of rock from cosmic rays and uses a water tank and outer detectors to protect against background neutrons. A suite of computational tools helps disentangle particle interactions and reject dark matter mimics.

The paper detailing the analysis will be released as a preprint on arXiv and submitted to the journal Physical Review Letters.