An international research collaboration has detected a possible trace of the weakly interacting massive particle, or WIMP, a leading candidate for dark matter.
The Institute for Basic Science (IBS) said the LUX-ZEPLIN (LZ) international collaboration reported observing a particle interaction that could be interpreted as a WIMP signal at an international particle physics conference held in Japan on the 1st.
The LZ collaboration is an international research team formed to directly detect WIMPs, dark matter candidate particles, and is led by Lawrence Berkeley National Laboratory under the U.S. Department of Energy. It brings together about 250 scientists and engineers from 39 institutions worldwide, with Korean participation from the team of Kim Young-duk, head of the IBS Center for Underground Physics.
Dark matter accounts for about 85% of all matter in the universe but has never been directly observed, because it neither emits nor reflects light. The WIMP, whose name means a weakly interacting massive particle, is one of the leading candidates for dark matter.
The difficulty is that WIMPs are far heavier than electrons or protons yet barely interact with light or other matter, making them hard to detect. On rare occasions, however, a WIMP can collide with an atomic nucleus inside a material, producing an extremely faint flash that can be converted into an electrical signal and captured by an ultra-precise detector.
To capture such an extremely faint signal, the LZ collaboration installed the LZ detector about 1.5 kilometers underground at the Sanford Underground Research Facility (SURF) in South Dakota. The researchers used photomultiplier tubes (PMTs) to capture the faint light and electrical signals generated when particles collide with about 10 tons of ultra-pure liquid xenon, then separated out signals resembling dark matter through detailed data analysis. A photomultiplier tube is a device that detects light, or photons, and converts it into an electrical signal.
Analyzing data collected over 220 days from March 2023 to April 2024, the researchers found one particle interaction in the region where a dark matter signal would be expected. If the signal was indeed produced by dark matter, the WIMP behind it is estimated to have a mass of at least 200 GeV/c², more than 200 times heavier than a proton. The result also suggests the WIMP may have interacted by transferring more energy to the nucleus, unlike the simple collisions that previous studies have mainly searched for.
The statistical significance of the signal came to 2.6 sigma. Sigma is a measure of whether an observed result is a coincidence, and 2.6 sigma means there is only a 0.5% probability that the signal occurred by chance.
Scientists say challenges remain, because the threshold for claiming the discovery of a new particle in physics is 5 sigma. Since this observation alone cannot confirm the existence of dark matter, researchers say further verification and observation are needed to improve statistical precision. The collaboration plans to continue its WIMP search at the Sanford facility and further test whether the signal is genuine.
Sam Eriksen, a senior research associate at the University of Bristol in the U.K. and lead author of the announcement, said the team analyzed a region of the data not covered in previous analyses and spent months closely reviewing every possible cause of the observed signal. Because the team has a precise understanding of the detector's characteristics and of signals that could be mistaken for dark matter, he said, even a single unusual signal that passes all checks carries significant meaning.
Rick Gaitskell, a professor at Brown University in the U.S. and a principal investigator on the LZ collaboration, said a dark matter signal is expected to appear and that the team is paying close attention because this signal was captured in a region where signals from other causes are very rare.
