China’s Underground JUNO Neutrino Lab Fires Up, Opening a Window to the Universe’s Ghost Particles

Photo credit: Yuexiang Liu / Institute of High Energy Physics
Deep beneath the granite hills of Guangdong, China, a big experiment has just started. The Jiangmen Underground Neutrino Observatory, or JUNO, began taking data on August 26, 2025 after more than 10 years of planning and construction.

JUNO is 700 meters underground to protect against cosmic radiation, and centered around a 35.4 meter diameter acrylic sphere filled with 20,000 tons of a special liquid called linear alkyl benzene. When a neutrino (especially an antineutrino from the adjacent nuclear reactors) hits this liquid, a faint blue light is produced. Around the sphere are 43,200 photomultiplier tubes, or PMTs, each with a super sensitive golden eye that catches these flashes and converts them into electrical signals that scientists can study. The sphere is floating in a 44 meter deep pool of ultra pure water, which is monitored by 2,400 additional PMTs to filter out cosmic muons. This is 20 times bigger than any previous detector of its kind.

Neutrinos are almost massless, electrically neutral and can go through matter as if it was not there. But they hold secrets that can change our understanding of physics. JUNO’s main goal is to determine if the third neutrino type, v₃, is heavier than the second, ν₂. This is more than just academic trivia; the answer can change our understanding of the universe, shed light on why matter dominates antimatter and how stars and galaxies formed. JUNO measures antineutrinos from the Taishan and Yangjiang nuclear power plants, 53 km apart, and catches about 45 of these interactions per day, building up an energy spectrum over time to reveal the mass hierarchy.

Unlike previous tests, its measurements are not affected by the Earth’s matter or entangled in complex parameter correlations. The detector’s energy resolution is unbeatable due to the transparency of the acrylic sphere and the sensitivity of its PMTs, with tiny 3-inch tubes filling the gaps between the larger 20-inch ones for maximum coverage. Early data from the commissioning runs, including a reactor neutrino event on August 24, 2025 with energies of 5.7 MeV and 2.2 MeV, show JUNO is already beating expectations. This precision can bring the neutrino oscillation parameters down to 1%, a big improvement over Daya Bay’s 4-7%.

Beyond its main goal, JUNO will eavesdrop on neutrinos from the Sun, giving us real-time views of solar activity. It can detect signals from distant supernovae, analyze geoneutrinos from Earth’s radioactive decay to understand mantle flow, and even look for sterile neutrinos or proton decay. With a 30-year life, JUNO has a future upgrade path to hunt for neutrinoless double-beta decay, a rare phenomenon that would prove whether neutrinos are their own antiparticles—a result that would change the particle physics textbooks.

This $376 million project has involved over 700 academics from 74 universities in 17 countries, including teams from the University of California, Irvine, the Technical University of Munich, and others in France, Germany, Italy, Russia and Taiwan. China is funding over 80% of the project and has drawn heavily from the Daya Bay experiment which ran from 2011 to 2020 and was globally acclaimed for discovering a critical neutrino oscillation phase. The UC Irvine team is the only US university with full participation, having worked on design, assembly and now data processing with funding from the National Science Foundation.

Building JUNO was no easy task. The acrylic sphere has walls 120mm thick and weighs over 600 tons, the largest of its kind ever made. Filling it with 20,000 tons of liquid scintillator and displacing 60,000 tons of water required centimeter-level precision to avoid structural collapse. Every component, from the cleanliness of the liquid to the calibration of the PMTs had to meet tight criteria. “It required not only new ideas and technology but also years of meticulous planning, testing and patience,” said JUNO’s head engineer Ma Xiaoyan.
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