For nearly a century, scientists have searched for dark matter, an invisible substance believed to account for approximately 85% of all matter in the universe. Its gravitational influence is evident across the cosmos, yet no experiment has directly detected dark matter. Determining what this mysterious substance is made of remains one of the biggest unanswered questions in modern physics.
New findings from the LUX-ZEPLIN (LZ) dark matter experiment have identified a single-particle interaction that is difficult to explain using known background signals produced by ordinary matter.
The result is not statistically significant enough to qualify as a discovery. However, researchers say it is the most compelling potential dark matter signal reported by LZ so far.
Giant dark matter detector located one mile underground
LZ is an international collaboration involving 250 scientists and engineers from 39 institutions. Managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), the experiment operates approximately one mile underground at the Sanford Underground Research Facility (SURF) in South Dakota.
At the center of the LZ detector are 10 tons of ultra-pure liquid xenon. The instrument was designed primarily to search for WIMPs, or weakly interacting massive particles, one of the leading candidates for dark matter.
The latest results were announced during a scientific lecture at the 2026 TeV Particle Astrophysics Conference in Japan. Research papers will also be posted to arXiv and submitted to Physical Review Letters.
Rick Gaitskell, a Brown University professor and LZ spokesperson, said: “We expect dark matter to appear, and it’s very intriguing that we can see this event in our data in an area where the competing background is so low.”
He added: “We don’t want to get ahead of ourselves with just one event. We don’t claim to have seen dark matter. But we saw something interesting and want to share it with the scientific community and get their opinions.”
LZ researchers examine a new region of the data
The LZ Collaboration analyzes its experimental results in batches. For this latest study, scientists examined 220 days of data collected between March 2023 and April 2024.
Researchers had previously analyzed the same dataset for subtle signals associated with the simplest types of WIMP interactions. In the new analysis, they expanded their search to include a broader range of possible WIMP interactions capable of depositing large amounts of energy in the detector.
Although LZ is highly sensitive to this type of event, its design also helps scientists determine whether particle interactions are caused by ordinary background processes rather than dark matter.
“This is a detailed study in an area that we had not investigated within this dataset, and we spent many months of additional effort understanding all possible causes of the background events,” said Sam Eriksen, a senior researcher at the University of Bristol in the UK and lead author of the study.
“Even one standout event like the one we discovered is important because we understand the detector and its environment so well. Dark matter events are expected to be extremely rare, so only a handful could mark the first detection of WIMP dark matter,” Eriksen added.
What could the unusual dark matter signal mean?
If dark matter produced the anomalous signal, the responsible WIMP would likely have a mass of at least 200 GeV/c2 (gigaelectronvolts). That would make it more than 200 times heavier than a proton.
The result could also point to a specific type of interaction between WIMPs and ordinary matter that extends beyond the simplest models typically used in dark matter searches.
Scientists are emphasizing caution. In particle physics, a result generally must reach a statistical significance of “5 sigma” before it can be considered a discovery. The new LZ result currently stands at 2.6 sigma.
According to the researchers, this corresponds to an approximately 0.5% chance that the unusual event was produced by a known background source.
Additional observations will be essential. As LZ collects more data, researchers will be able to determine whether the statistical significance of the potential signal increases or whether it eventually disappears.
LZ has already produced the world’s largest dataset for dark matter searches. The experiment will continue collecting WIMP data at SURF, providing scientists with stronger statistics and more opportunities to investigate the potential signal.
How the LZ experiment separates dark matter from background noise
The LZ experiment searches for dark matter by detecting flashes of light produced when particles deposit energy inside the detector.
The challenge is that similar particle interactions can occur in ordinary materials. To prevent these events from being mistaken for dark matter, LZ uses multiple layers of shielding, detection technology, and data analysis.
The experiment’s underground location provides one of its first lines of defense. Approximately one mile of rock above the facility blocks much of the cosmic-ray radiation that reaches Earth from space. A surrounding water tank and additional outer detectors help shield the central detector from background neutrons.
Researchers also use advanced computational methods to distinguish different types of particle interactions and reject events that resemble the expected dark matter signature.
The unusual event has attracted attention because, so far, it has not shown the typical signs of a background problem that scientists often identify when examining outliers more closely.
“Outlier events in the data are not unexpected, but when you look more closely, they usually stand out as some sort of background,” said Aaron Manarasay, a physicist at Berkeley Lab and chair of the LZ institutional board.
“This is the first experiment I’ve worked on that has a plausible outlier in every respect. Of course, we’re still considering whether there are rare background mechanisms we may have missed, but it’s exciting to think this could be the first hint of a dark matter observation,” Manarasay added.
For now, a single unexplained event is not enough to confirm the detection of dark matter. However, because the signal appeared in a region where dark matter interactions are expected and has withstood extensive analysis, researchers believe it deserves close attention as LZ continues collecting data.
International support for the dark matter search
LZ is supported by the U.S. Department of Energy’s Office of Science, the Office of High Energy and Nuclear Physics, and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility.
Additional support comes from the UK Science and Technology Facilities Council, the Portuguese Foundation for Science and Technology, the Swiss National Science Foundation, the Australian Research Council Centre of Excellence for Dark Matter Particle Physics, and the Korea Institute of Basic Science.
Thirty-nine higher education and advanced research institutions support the LZ experiment. The LZ Collaboration also acknowledges support from the Sanford Underground Research Facility.
Source: www.sciencedaily.com