LUX-ZEPLIN Experiment Reports Anomalous Event in Dark Matter Search, More Data Needed for Verification

On September 1, the LUX-ZEPLIN (LZ) dark matter direct detection experiment released a new analysis result: researchers recorded a single-particle interaction event in the data whose characteristics could be interpreted as a Weakly Interacting Massive Particle (WIMP) signal, but the current evidence is insufficient to confirm a dark matter discovery.

The LUX-ZEPLIN main detector is shown in a surface laboratory before being installed underground. (Matthew Kapust/Sanford Underground Research Facility)

The LZ experiment involves approximately 250 scientists and engineers from 39 institutions, with the detector located nearly one mile underground at the Sanford Underground Research Facility in South Dakota, USA, and managed by the U.S. Department of Energy's Lawrence Berkeley National Laboratory. The experiment uses 10 tons of ultra-pure liquid xenon as the detection medium, primarily to search for WIMPs, the theoretical dark matter candidate particles.

These results are based on 220 effective working days of data collected between March 2023 and April 2024. The research team stated that this analysis is no longer limited to the simplest WIMP interaction model but has been extended to include interaction types that could deposit more energy in the detector. The results were reported at the 2026 TeV Particle Astrophysics conference held in Japan, and the paper will be posted on arXiv and submitted to Physical Review Letters.

To search for dark matter, LZ uses photomultiplier tubes (shown here before installation into the detector) to capture light produced by particle interactions. (Matthew Kapust/Sanford Underground Research Facility)

The researchers noted that the event appeared in a region where a dark matter signal might be expected, with a correspondingly very low background level. However, with only one candidate event, the result has a significance of 2.6σ, below the 5σ standard typically used in the physics community to confirm a major discovery. In other words, there remains approximately a 0.5% probability that the event could be explained by known background radiation.

If the anomalous event is indeed caused by dark matter, the corresponding WIMP mass could be at least 200 GeV/c², more than 200 times the mass of a proton, and could imply a form of interaction between WIMPs and ordinary matter beyond the simplest models.

LZ uses a cylindrical chamber filled with liquid xenon to search for dark matter. The chamber is surrounded by additional layers to detect or block background particles (left). When a WIMP collides with a xenon atom (right), it produces a flash of light and electrons. These flashes are detected at the top and bottom of the liquid xenon chamber. An electric field drifts the electrons to the top of the chamber, where they produce a second flash. (Greg Stewart/SLAC National Accelerator Laboratory)

The LZ detector searches for dark matter by capturing the light flashes and electron signals produced when particles deposit energy in the liquid xenon. To reduce interference, the experiment uses underground rock to shield against cosmic rays, and employs a water tank, external detectors, and computational analysis tools to reject background signals from ordinary matter interactions. A team from the U.S. Department of Energy's Brookhaven National Laboratory also developed and provided 20 tons of gadolinium-loaded liquid scintillator for key detector components, helping to identify and reject spurious signals.

The LZ collaboration stated that future data will determine whether this anomalous signal gradually strengthens or disappears as statistics increase. The experiment continues to collect WIMP search data at the Sanford Underground Research Facility to further improve the statistical sensitivity of dark matter direct detection.