A Lightweight Particle That Is Difficult to Capture

The four-minute elevator ride—better described as a descent— takes visitors 700 meters underground to a vast cavern beneath thick granite layers. As you go deeper into the underground facility, the environment becomes cleaner—protective clothing, including headcovers and shoe covers, is worn to minimize dust contamination. At the center lies a 35-meter-diameter acrylic sphere filled with a clear liquid. It is densely covered with thousands of copper-colored photomultipliers that resemble watchful eyes, all surrounded by a massive water pool.

Imagine it: we are in Jiangmen, Guangdong Province, southern China, at the Jiangmen Underground Neutrino Observatory (JUNO). It is pitch black far below the surface. The hoped for sign of light occurs when a neutrino—an electrically neutral elementary particle—interacts with the scintillating material inside the sphere, causing the emission of a photon. The photomultiplier tubes amplify this faint light to measurable levels. A delayed gamma photon confirms that the light flash was indeed triggered by a neutrino. Since starting measurements in August 2025, this event has occurred about 50 times per day.

Neutrinos are produced by the decay of radioactive elements. Their natural sources include the cosmos, the sun, the Earth’s atmosphere, and the planet’s interior rocks, classified as cosmic, solar, atmospheric, and geoneutrinos respectively. Because of their extremely low interaction rate, neutrinos are very long-lived and stream freely across the universe, capable of passing through the entire Earth unimpeded. “There are still neutrinos from the Big Bang, the universe’s origin over 13 billion years ago,” says Lachenmaier. For the JUNO project, however, reliance wasn’t placed solely on the low-level natural neutrino background. “The Jiangmen site was chosen for its proximity—about 50 kilometers—to two nuclear power plants, providing JUNO with a particularly abundant flux of reactor neutrinos,” he adds.

Tübingen has been involved since JUNO’s inception a decade ago. The international collaboration is led by the Institute of High Energy Physics of the Chinese Academy of Sciences and includes more than 700 scientists from 74 institutions in 17 countries and regions. After around ten years of construction, JUNO, the world's largest and most precise neutrino detector of the latest generation, was put into operation. “China leads and funds the majority of JUNO; it is a prestigious basic research project. About 40 percent of collaborators are non-Chinese, and many Chinese researchers have experience working in Western countries,” notes Lachenmaier, who finds the scientific cooperation open and productive. In such a large collaboration, one might feel like a small cog, he says, but all parts must mesh perfectly. The measurement equipment for the 17,000 photomultiplier tubes used in JUNO was manufactured in Tübingen. After shipment to China, four optical laboratories were set up in a factory hall and each photomultiplier was quality-tested remotely from Tübingen. “The best tubes were installed in the acrylic sphere, while the less favorable ones were placed in the outer regions,” Lachenmaier explains.