Physicists using the LUX-ZEPLIN (LZ) dark matter detector, buried deep in a former South Dakota gold mine, say they have recorded a single particle interaction that they cannot easily explain. They are not claiming to have found dark matter, but the event sits in a region where a real signal would be expected.
A detector built for rare events
LZ is an international collaboration of about 250 scientists and engineers from 39 institutions. The detector holds 10 tons of ultrapure liquid xenon and sits 1.6 kilometers below ground at the Sanford Underground Research Facility in Lead, South Dakota, where the rock above blocks most cosmic noise. It is designed to catch weakly interacting massive particles, or WIMPs, one of the leading candidates for dark matter, the invisible substance that makes up most of the mass in the universe.
One event in a quiet corner
The team examined 220 days of data collected between March 2023 and April 2024, this time searching a wider range of energies than earlier studies covered. In that data they found a single event in a part of the detector that had not been examined before. “It wouldn't take many [events] to represent the first real detection of dark matter,” said Carmen Carmona, who leads the LZ group at Penn State. She said the team spent months ruling out background sources before sharing the result. If the event was caused by dark matter, the particle would likely weigh at least 200 GeV, or more than 200 times the mass of a proton.
Well short of discovery
The signal carries a statistical weight of 2.6 sigma, meaning there is roughly a 0.5 percent chance it is a fluke of known backgrounds. Physicists usually demand 5 sigma before declaring a discovery. “We are not claiming to have seen dark matter,” said Rick Gaitskell of Brown University, the LZ spokesperson. “But we have seen something interesting that we want to share with the scientific community.” The results will be submitted to the journal Physical Review Letters, and the team plans to keep running the detector to gather more data.