A Northwestern University physicist helped detect a single mysterious particle interaction that scientists cannot explain with any known source, according to results published Tuesday, Sept. 1, by the LUX-ZEPLIN experiment.

The collaboration described the finding as the most compelling hint yet of dark matter, the invisible material that makes up roughly 85% of the universe's total matter. No one has ever directly detected it.

Eric Dahl, a professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences, co-authored the study and called the result the standout moment of his career.

"In the 20 years that I've been involved in the search for dark matter, this is the most interesting single event that I've seen," Dahl said. "Over the past year, we have spent a lot of time poring over ways to explain this event. After doing the math, we haven't found anything with even a 1% chance of creating something like this signal."

The LUX-ZEPLIN (LZ) collaboration includes 250 scientists and engineers from 39 institutions. The detector sits nearly a mile underground at the Sanford Underground Research Facility in South Dakota. It is managed by the U.S. Department of Energy's Lawrence Berkeley National Laboratory, known as Berkeley Lab.

What the detector found

The LZ detector is filled with 10 metric tons of ultrapure liquid xenon cooled below minus 108 degrees Celsius. When a particle strikes a xenon atom, it produces flashes of light that researchers can measure.

Researchers analyzed 220 days of data collected between March 2023 and April 2024, according to Berkeley Lab. They found one anomalous event, designated LZ.230616 for the date it occurred. The interaction deposited far more energy than a standard dark matter candidate, called a WIMP (weakly interacting massive particle), would typically produce.

Statistical analysis put the result at 2.6 sigma, meaning there is roughly a 0.5% chance the event came from a known background source. That falls short of the 5-sigma threshold physicists require to claim a discovery.

If the event was caused by dark matter, the particle responsible would have a mass of at least 200 gigaelectronvolts (GeV), or more than 200 times the mass of a proton, Berkeley Lab reported. It would also suggest a type of interaction beyond the simplest dark matter models.

Dahl's role: ruling out false alarms

At Northwestern, Dahl and his team worked to distinguish a potential dark matter signal from ordinary phenomena that can mimic one. His group spent significant time tracking radon, a naturally occurring radioactive gas, inside the detector. Radon decay is one of the largest sources of false signals.

The collaboration is not claiming a discovery. The analyzed data represents only about one-quarter of what LZ will collect during its lifetime. A dataset roughly twice the size of the one used to find LZ.230616 is already being calibrated and analyzed, according to the University of California, Santa Barbara.

What comes next

The study authors presented the work at the 2026 TeV Particle Astrophysics conference in Japan. They plan to submit the paper to Physical Review Letters.

LZ will continue collecting data at the South Dakota facility. That larger dataset will either confirm LZ.230616 as evidence of dark matter or rule it out.