A detector buried deep underground has recorded an event that researchers have not yet been able to link to a known cause.
The LUX-ZEPLIN experiment, or LZ, is directly searching for the particles that could make up dark matter. This invisible matter is inferred from its gravitational effects on galaxies and other celestial objects. Its nature remains unknown, as none of its hypothesized particles has yet been detected with certainty.
Photomultiplier array of the LUX-ZEPLIN detector, designed to capture the very faint flashes of light produced in liquid xenon.
Credit: Matthew Kapust / Sanford Underground Research Facility
To try to detect one, LZ uses around 10 tonnes of highly pure liquid xenon. The detector is located nearly 1.5 km underground in South Dakota. This depth blocks much of the particle radiation from space that could produce false signals.
When a particle strikes a xenon atom, it can trigger tiny emissions of light and release electrons. Researchers analyze these traces to determine their origin. They are particularly looking for WIMPs, hypothetical massive particles that would interact very rarely with ordinary matter.
The new analysis covers 220 days of measurements collected between March 2023 and April 2024. It explores interactions capable of depositing more energy than those targeted in previous analyses. One event stands out: after months of checks, the team found no known background noise that could easily explain it.
When a WIMP particle collides with a xenon atom, the atom emits a flash of light and electrons. The light is detected at the top and bottom of the liquid xenon chamber. An electric field drives the electrons toward the top of the chamber, where they generate a second flash of light.
Credit: Greg Stewart, SLAC National Accelerator Laboratory
The result reaches 2.6 sigma, a statistical measure used to estimate how distinct a result is from simple chance. In particle physics, a discovery generally requires 5 sigma. According to the team, known background noise would have about a 0.5% chance of producing a result at least this unusual.
If the event was genuinely caused by a dark matter particle, it would probably have a mass greater than about 200 times that of a proton. It would also correspond to a broader type of interaction than those in the simplest models usually tested by LZ. For now, this scenario remains one possibility among several.
Researchers remain especially cautious because they have only one event. A rare phenomenon in the detector that is still poorly understood could also be responsible. The team is therefore working on calibrations and possible sources of noise in this energy range.
What happens next will depend mainly on the new data. LZ continues to record interactions in its xenon tank. If other similar events appear, the statistical significance of the signal could increase. If it remains isolated, its importance could instead diminish as more measurements accumulate.