The search for dark matter - the invisible substance that makes up the vast majority of mass - remains one of the biggest mysteries of modern science.
Now an international physics experiment involving researchers from the University of Liverpool has made an intriguing and surprising observation.
New analysis from the LUX-ZEPLIN (LZ) experiment has recorded a single particle interaction that researchers have great difficulty explaining with known background processes and could potentially have been caused by a WIMP (weakly interacting massive particles).
Whilst the result does not yet meet the statistical threshold required to claim a discovery, it is the most compelling hint of dark matter reported by the experiment to date.
LZ is an international collaboration of 250 scientists and engineers from 39 institutions, including researchers from the University of Liverpool, who have been involved in the experiment dating back to 2014.
The latest LZ results were presented in a scientific talk at the 2026 TeV Particle Astrophysics conference in Japan. The paper will be released on the online repository arXiv and submitted to the journal Physical Review Letters.
The LZ collaboration studies experimental data in batches. In the new result, researchers analyzed 220 live days of data collected between March 2023 and April 2024. The collaboration had previously searched this dataset for faint signals from the simplest kinds of WIMP interactions. The new analysis searched for a broader range of possible WIMP interactions that could deposit more energy in the detector. LZ is particularly sensitive to such signals while also minimizing false positives.
If the anomalous event was caused by dark matter, the WIMP that generated it would likely have a mass of at least 200 GeV/c2 (gigaelectronvolts), 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 model. The LZ results have not reached "5-sigma" significance, the statistical threshold considered a discovery in physics. The new analysis is 2.6 sigma, meaning there is approximately a 0.5% chance that the event could be explained by known backgrounds.
The team at Liverpool, led by Professor Sergey Burdin, maintain and operate the Optical Calibration System for the LZ Outer Detector, which was produced with support from the Detector Development and Manufacturing Facility. The Outer Detector plays an essential role in identifying, suppressing and understanding nuclear-recoil backgrounds.
Dr Ewan Fraser leads the LZ Data Quality group, which is crucial for identifying and understanding detector effects, as well as studies of the neutron background, which is the main physics background for this type of event.
Dr Billy Boxer, while a PhD student in our group, laid important groundwork for this analysis, and our PhD student Megan Carter, supervised by Dr Ewan Fraser, has played a major role in carrying it out.
Commenting on these latest results, Professor Burdin said: "I joined the LZ experiment in 2014 because I believed that this type of experiment offered one of the most promising routes to discovering dark matter. It is therefore extremely exciting to see an event like this.
"At the same time, we have to be cautious. It could still turn out to be an unusual background event that we do not yet fully understand. However, it has several striking features that are consistent with what we might expect from a dark matter interaction.
"What makes the event particularly intriguing is that it does not resemble the type of dark matter interaction we had considered most likely, which would predominantly produce low-energy nuclear recoils. If this signature is confirmed, it will provide important new information about the nature and properties of dark matter, with significant implications not only for direct-detection experiments such as LZ, but also for dark matter searches at particle colliders and through signals from space."
LZ searches for dark matter by looking for signature flashes of light from energy deposited in the detector. The collaboration leverages multiple methods to prevent or account for particle interactions caused by normal matter. This includes the mile of rock that shields the detector from cosmic rays from space, a water tank and outer detectors that protect the central detector from background neutrons, and a suite of computational tools that disentangle particle interactions and reject dark matter mimics.
Professor Burdin added: "LZ still has a substantial amount of data to analyse and continues to collect more. Moreover, we are designing an even larger next-generation detector, XLZD, which could potentially be hosted in the UK. We will be searching for additional events like this, and I am confident that, with more data, we will ultimately understand their origin."
LZ is an international collaboration of 250 scientists and engineers from 39 institutions. The detector is managed by the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and operates nearly one mile below ground at the Sanford Underground Research Facility (SURF) in South Dakota. The experiment uses 10 tonnes of ultrapure liquid xenon to search for dark matter and is optimized to look for WIMPs, or weakly interacting massive particles.
Main image caption: Looking up into the LZ outer detector, used to veto radioactivity that can mimic a dark matter signal. Credit: Matthew Kapust/Sanford Underground Research Laboratory
LZ uses a cylindrical chamber full of liquid xenon to search for dark matter. It is surrounded by additional layers to detect or block background particles (left).
