Only two months after its completion, the next-generation neutrino detector JUNO (Jiangmen Underground Neutrino Observatory) in south-eastern China, to which the INFN makes a decisive contribution, has already achieved extraordinary precision, determining some fundamental parameters of solar neutrino oscillation with unprecedented accuracy. The results, submitted to Chinese Physics C and available on arXiv, were presented today, November 19, by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences.
After more than a decade of design and construction by the international Collaboration, JUNO is the first large-scale, high-precision neutrino detector to come into operation, and in just 59 days of data taking, from August 26 to November 2, 2025, it has already met the project expectations. The measurement of solar neutrino oscillation parameters known as θ12 and Δm221 is in fact more precise by a factor of 1.6 and 1.8 compared with that obtained from the combined analysis of all previous measurements in the sector.
These parameters, originally determined using solar neutrino data, can also be measured with high precision through reactor antineutrinos, as JUNO has done in its first measurement. In the past, the results obtained separately by the two methods showed a slight discrepancy of 1.5 sigma: a “tension” that may suggest the presence of new physics. The measurement with antineutrinos just carried out by JUNO confirms this tension, which the experiment will be able to verify definitively when it also performs the measurement of solar neutrinos from ⁸B.
“Achieving such precision within only two months of operation shows that JUNO is performing exactly as expected”, commented Yifang Wang, head of the JUNO project. “With this level of precision, JUNO will soon determine the ordering of neutrino masses, subject the three-flavour oscillation scheme to accurate tests, and also look for possible signs of new physics”.
“The scientific result announced today demonstrates how fruitful the decade-long commitment of the JUNO Collaboration has been to the assembly of a detector based on a variety of cutting-edge technologies, which will dominate the neutrino physics landscape in the coming years, providing results of extreme precision”, added Gioacchino Ranucci, JUNO international deputy coordinator and researcher at the INFN and the University of Milan. “Many factors have contributed to this success: of fundamental importance has been the convergence of experience and expertise in liquid-scintillator detectors and related analysis techniques, contributed by research groups from all over the world”.
JUNO is a highly internationalised project, managed in China by the IHEP Institute with which the INFN boasts a long tradition of cooperation, and involves more than 700 researchers from 75 institutions in 17 countries and regions. The INFN participates through the divisions of Catania, Ferrara, Milan, Milan Bicocca, Padua, Perugia, Roma Tre and with the Frascati National Laboratories. “As Chair of the JUNO Institutional Board, I am proud to see this global effort reach such an important milestone. The success of JUNO reflects the commitment and creativity of the entire international community”, declared Marcos Dracos, researcher at the University of Strasbourg and CNRS/IN2P3 in France.
The JUNO experiment was proposed in 2008 and approved by the Chinese Academy of Sciences (CAS) and the Guangdong Province in 2013. INFN joined the initiative in 2014, being the first among foreign institutes currently involved. Construction of the underground laboratory began in 2015, while installation of the detector, which started in December 2021, was completed in December 2024, immediately followed by filling first with ultra-pure water, then with liquid scintillator.
The heart of the experiment is a 35.4 m-diameter acrylic vessel containing 20,000 tonnes of liquid scintillator, located at the centre of a 44-metre-deep water pool within an underground experimental hall. The acrylic vessel is supported by a 41.1-metre-diameter stainless steel lattice shell, which also houses 20,000 20-inch photomultiplier tubes (PMTs), 25,600 3-inch PMTs, along with the rest of the instrumentation including front-end electronics, cables, coils for compensating the Earth’s magnetic field, and light separation panels. All PMTs operate simultaneously to capture the scintillation light produced by neutrino interactions inside the scintillator and convert it into electrical signals.
During construction, numerous unprecedented milestones were achieved, such as a high-performance PMT characterised by innovative design, both in structure and electronic amplification. Other technological achievements include the development of explosion-proof and waterproof PMT housings; the aforementioned high-efficiency purification system producing radio-pure scintillator with a light attenuation length exceeding 20 metres; and innovative underwater electronics with aerospace-grade reliability using commercially available components.
With its extraordinary sensitivity, JUNO will determine the neutrino mass hierarchy and measure oscillation parameters with an accuracy of less than 1%. It will also study solar, atmospheric, supernova and geoneutrinos, and search for phenomena beyond the Standard Model. Designed for a scientific lifetime of about 30 years, the experiment can also be upgraded to investigate neutrino-less double beta decay, the absolute scale of neutrino mass and verify whether neutrinos are Majorana-type particles.
