The underground neutrino observatory “Jiangmen Underground Neutrino Observatory (JUNO)” near the city of Jiangmen in the southern Chinese province of Guangdong, in whose preparation researchers from the cluster of excellence PRISMA⁺ of the Johannes Gutenberg University Mainz (JGU) are also involved, has completed filling the neutrino detector with 20,000 tons of liquid scintillator and is now starting data acquisition. After more than ten years of construction, JUNO is the first experiment of a new generation of world-leading neutrino experiments that can now go into operation. In recent months, initial tests have shown that the detector’s most important performance characteristics meet or even exceed the design specifications. This will enable JUNO to investigate one of the currently central questions of Particle Physics: the ordering of neutrino masses, and thus whether the third neutrino mass state (ν₃) is heavier than the second (ν₂).

Prof. Yifang Wang, member of the Chinese Academy of Sciences, the Institute of High Energy Physics Beijing (IHEP) and spokesperson of the JUNO Collaboration, says: “The completion of the JUNO detector’s filling phase and the start of data acquisition represent a historic milestone. For the first time, a detector of this magnitude and precision is in operation, dedicated exclusively to neutrinos. JUNO will help us answer fundamental questions about the nature of matter and the universe.”

JUNO was built in a specially created underground laboratory at a depth of 700 meters. The detector detects antineutrinos produced 53 kilometers away from the nuclear power plants in Taishan and Yangjiang. JUNO measures their energy spectrum with unprecedented precision. Unlike other experimental approaches, JUNO’s method for determining the neutrino mass ordering is independent of matter effects in the Earth and the exact value of other neutrino oscillation parameters. JUNO will also improve the precision of several of these parameters by orders of magnitude. JUNO is also an extremely sensitive observatory for solar, supernova, atmospheric, and geoneutrinos and will also be used for the search for sterile neutrinos and proton decay.

JUNO was proposed in 2008 and approved in 2013 by the Chinese Academy of Sciences and Guangdong Province. Construction of the underground laboratory began in 2015. Detector installation started in December 2021 and was completed in December 2024. Subsequently, 60,000 tons of ultra-pure water were filled within 45 days, with liquid levels inside and outside the central acrylic sphere maintained to within a few centimeters and flow rates controlled with an uncertainty below 0.5% to ensure the structural integrity of the detector. Over the following six months, 20,000 tons of liquid scintillator were filled into the acrylic sphere, displacing the water. Strict requirements for purity, optical transparency, and extremely low radioactivity for water and scintillator had to be met. In parallel, the collaboration started commissioning and optimizing the detector, which, after the completion of the filling phase, allows for a direct transition to full JUNO operation.

The core of the JUNO experiment is the central liquid scintillation detector for neutrino detection with an unprecedented effective mass of 20,000 tons. This is embedded in a 44-meter-deep water pool. A 41-meter-high stainless steel structure surrounds the detector, supporting the 35.4-meter acrylic sphere, the scintillator, 20,000 photomultiplier tubes (PMTs) with 20 inches (50 cm) diameter, 25,600 PMTs with 3 inches (7.5 cm) diameter, the front-end electronics, cabling, and magnetic compensation coils. All PMTs together collect the scintillation light from neutrino reactions and convert the detected photons into electrical signals.

Prof. Xiaoyan Ma, Chief Engineer of JUNO, summarizes this as follows: “The construction of JUNO was a journey full of extraordinary challenges. It required not only new ideas and technologies but also years of careful planning, testing, and perseverance. Adhering to the strict requirements for purity, stability, and safety demanded the dedication of hundreds of engineers and technicians. Their team spirit and integrity transformed a bold design into a functional detector, now ready to open a new window to the neutrino world.”

JUNO is operated by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences and unites over 700 researchers from 74 institutions in 17 countries. From Germany, six research units are involved in the experiment – with support from the German Research Foundation (DFG) – including the work groups of Prof. Michael Wurm and Prof. Livia Ludhova, both members of the cluster of excellence PRISMA⁺ of the Johannes Gutenberg University Mainz. “JUNO is the result of long-standing international cooperation,” emphasizes Prof. Livia Ludhova, who is a member of the JUNO Executive Committee. “Our teams have contributed important building blocks to the current success: with the OSIRIS pre-detector for ensuring the radioactive purity of the scintillator during detector filling, with sensitivity studies, and with the analysis of the first data now taken during commissioning, all in close collaboration with our co-workers in China. It is very satisfying to see how our combined expertise now comes together in a detector that will serve the global physics community for decades.”

JUNO is designed for a scientific lifetime of up to 30 years. In the future, the detector offers the possibility for an upgrade to conduct a world-leading search for neutrinoless double-beta decay. Such an upgrade would explore the absolute mass scale of neutrinos and test whether neutrinos are Majorana particles – a fundamental question that connects Particle Physics, astrophysics, and cosmology.