Has the unknown entity that made stars and galaxies revealed its identity? The LUX-ZEPLIN (LZ) international collaboration, which includes Korea, said on the 1st at an international particle physics conference in Japan that it has detected traces of WIMP, a leading dark matter candidate.
Scientists say only 15% of the universe's matter emits its own light and can be observed. The remaining 85% does not emit light and is invisible dark matter. They say this pulled matter together to form stars and galaxies, because gravity, the force that attracts particles to each other, is not enough on its own to make stars.
◇ Trace of a particle heavier than a proton captured
The LZ international collaboration said it captured a flash in the LZ detector at the Sanford Underground Research Facility in South Dakota, where dark matter interacted with a xenon particle. It was an event in which energy of 248 keV (kiloelectronvolts) occurred in the detector. The LZ team believes the mass of the dark matter particle that caused this flash must be at least 200 GeV (gigaelectronvolts). One GeV is roughly the mass of a proton.
WIMP, currently considered the top candidate for dark matter, is an English acronym meaning "weakly interacting massive particle." It means a particle heavy enough to have gravity while interacting very little with other matter. WIMPs are much heavier than electrons or protons, but they interact very little with light or other matter, making them hard to detect. Only very rarely, when they collide with an atomic nucleus in matter, can the extremely faint flash be converted into an electrical signal and captured by an ultra-precise detector.
The LZ detector is a device filled with about 10 tons of ultra-high-purity liquid xenon about 1.5 kilometers underground. Its deep underground location can block other particles coming from space. Inside the tank containing xenon are photomultiplier tubes (PMTs) that capture faint light and electrical signals generated when particles collide. They are ultra-high-sensitivity optical sensors that detect light at the single-photon level, the minimum unit of light, and amplify and convert it into electrical signals by more than millions of times.
◇ Not yet at the level of a definitive discovery
The team analyzed data collected over 220 days from Mar. 2023 to Apr. 2024 and captured one particle interaction in the region where a dark matter signal is expected to appear. The team estimated that if this signal was indeed produced by dark matter, then the mass of the WIMP would be more than 200 times heavier than a proton in the atomic nucleus.
Rick Gaitskell, a professor at Brown University and head of the LZ collaboration, said, "We are paying attention because this signal was captured in a region where a dark matter signal is expected and where signals from other causes are very few."
However, the results are not yet at a level recognized as a scientific discovery. In physics, a new discovery requires a discrepancy level five times the uncertainty, or 5 sigma. Generally, if an experiment's confidence level is 3 sigma (99.7%), it falls into the "hint" category, and if it is 5 sigma (99.99994%) or higher, it is recognized as a "discovery." The signal currently captured by LZ is at the 2.6-sigma (99.07%) level, which means the probability that it arose by chance is about 1 in 200.
But Sam Eriksen, the first author of the paper and a senior researcher at the University of Bristol, said, "We newly analyzed a data region not covered in previous analyses, and we carefully examined all possible causes for months to identify what produced the observed signal," adding, "Because we precisely understand the detector's characteristics and signals that could be mistaken for dark matter, even a single anomalous signal that passes all checks has important significance."
◇ Hopes for discovery in other detectors as well
The encouraging point is that this analysis dealt with only one-third of the data LZ has already collected. The team said it has secured the largest dataset among dark matter search experiments to date and will continue the WIMP search at the Sanford Underground Research Facility in the United States to further verify the authenticity of this signal.
Other dark matter detection facilities around the world, including in Italy, China and Korea, also have their own data. Korea operates the Yemi Lab, which studies dark matter and cosmic particles, 1,000 meters underground on Mount Yemi in Jeongseon-gun, Gangwon Special Self-Governing Province. If several more events are observed in an energy range like this one, the confidence level of a WIMP discovery could be raised to 5 sigma. That could completely change our understanding of physics and the universe.
Researchers led by Director General Kim Young-deok of the Institute for Basic Science (IBS) underground experiment group also took part in this study. Kim said, "IBS is the only Asian research institution to participate, and we are also working on the next-generation experiment, XLZD," adding, "Korea could play a more leading role in liquid xenon detector experiments, which are showing strong performance in WIMP observations."
Douglas Leonard, an IBS research fellow, said, "Something we do not yet understand has appeared, and whichever way it turns out, we are learning and discovering something new."
The results have not yet been published as a formal paper and were first posted on the preprint site arXiv. The team said the paper has been submitted to the international journal Physical Review Letters.
Dark matter and dark energy are hot research topics in astronomy. The Nancy Grace Roman Space Telescope, launched by the United States on the 30th of last month, also plans to track dark energy and dark matter.
Scientists say only 5% of the energy in the universe is in matter that emits light and can be observed, and 70% is dark energy with a repulsive force. The remaining 25% is called dark matter, which does not emit light but pulls objects together. In terms of matter alone, dark matter is 85% and ordinary matter is 15%.
References
LUX-ZEPLIN (2026), https://lz.lbl.gov/wp-content/uploads/sites/6/2026/08/LZ_Preprint_260901_Dark_Matter_EFT_N