On 26 August 2025, a physics experiment started at the Jiangmen Underground Neutrino Observatory (JUNO). The international collaboration of scientists involved in the experiment, which includes employees of the Laboratory of Nuclear Problems at JINR, officially announced the completion of filling JUNO’s central detector with 20 000 tonnes of liquid scintillator and began registering physics events. The aim of the experiment is to determine neutrino mass ordering and to precisely measure the parameters of neutrino oscillations.

In the south of China, the construction of the Jiangmen Underground Neutrino Observatory completed. This is the world’s largest experiment of its kind and the first new-generation project to reach the data collection stage. The experimental hall of the JUNO Facility is shielded from cosmic muons by 700 metres of rock. The detector, filled with 20 000 tonnes of liquid scintillator, is equipped with 20 000 twenty-inch and about 25 000 three-inch photomultipliers for registering the interactions of antineutrinos emitted by a powerful reactor complex. The average distance between the reactors and the JUNO detector is about 53 km, which ensures maximum sensitivity of the experiment to determining neutrino mass ordering – the collaboration’s key task. With the participation of more than 700 scientists from 17 countries, JUNO will study neutrinos from supernovae, the Sun, and the Earth as well.

The Jiangmen Underground Neutrino Observatory successfully completed the filling of the central detector with twenty thousand tonnes of liquid scintillator on 26 August 2025 and began collecting physics data. Preliminary technical data taking showed that the detector’s key performance indicators met or exceeded design expectations. This will allow JUNO to answer one of the most important questions in modern particle physics – to determine neutrino mass ordering: is the third mass state (ν₃) heavier than the second (ν₂)?

Professor, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, JUNO Spokesperson Yifang Wang noted: “Completing the filling of the JUNO detector and starting data taking marks a historic milestone. For the first time, we have in operation a detector of this scale and precision dedicated to neutrinos. JUNO will allow us to answer fundamental questions about the nature of matter and the universe”.

Unlike other experiments, JUNO does not rely on the effects of neutrino propagation in the Earth’s matter to determine neutrino mass ordering and is largely free from the associated uncertainties. Moreover, JUNO will increase the precision of determining the fundamental parameters of the lepton sector of the Standard Model by more than 10 times and will become a key tool in the study of neutrinos from supernovae, the Sun, and the Earth, as well as in the search for sterile neutrinos and proton decay.

JUNO Chief Engineer Xiaoyan Ma emphasised that the construction of JUNO turned out to be an extremely complex task calling not only for new ideas and technologies, but also for many years of careful planning, testing, and hard work. “Meeting the stringent requirements of purity, stability, and safety called for the dedication of hundreds of engineers and technicians. Their teamwork and integrity turned a bold design into a functioning detector, ready now to open a new window on the neutrino world,” Xiaoyan Ma said.

JUNO is designed for a scientific lifetime of up to 30 years, with the possibility of upgrading to a world-leading experiment in the search for neutrinoless double-beta decay. Such an upgrade would allow studying the absolute neutrino mass scale and testing whether neutrinos are Majorana particles. This adresses the fundamental questions at the intersection of particle physics, astrophysics, and cosmology. It could radically change our understanding of the universe.

“The landmark achievement that we announce today is also a result of the fruitful international cooperation ensured by many research groups outside China, bringing to JUNO their expertise from previous liquid scintillator set-ups. The worldwide liquid scintillator community has pushed the technology to its ultimate frontier, opening the path towards the ambitious physics goals of the experiment,” JUNO Deputy Spokesperson, a Professor at the University of Milan and INFN-Milan Gioacchino Ranucci noted.

“Photomultipliers are the eyes of a detector. JUNO uses PMTs with record sensitivity to detecting light. JINR employees developed and applied a unique methodology for studying and testing them. Our employees developed and commissioned power systems for large and small PMTs, as well as a muon veto detector. They significantly contributed to developing and creating the TAO Detector and assembling many other systems of the experiment,” Deputy Head of the JUNO Project at JINR, Head of the DLNP JINR Sector of Research Methodology Nikolay Anfimov said.

“In addition, we have established a computing centre for data storage, modelling, reconstruction, and analysis in Dubna. Our team is actively involved in the analysis of JUNO’s experimental data. Currently, all efforts are focused on studying reactor antineutrino oscillations registered in our detector,” Deputy Head of the JUNO Project at JINR, Head of the DLNP JINR Reactor Neutrino Sector Maxim Gonchar added.

Head of the JUNO Project at JINR, DLNP JINR Deputy Director Dmitry Naumov noted that the Institute has been involved in the project since the very start: “JUNO is the result of international effort. Our team has contributed to many of its key steps. Besides our institute, two other groups from Russia are actively participating in the project: the Skobeltsyn Institute of Nuclear Physics of Moscow State University and the Institute for Nuclear Research of the Russian Academy of Sciences. Working side by side with colleagues in China and around the world has been both a challenge and an honour. I am glad to see that these efforts have resulted in the creation of a detector that will serve the world of science for decades to come”.

Based on an article by the Scientific Communications Group of the
Laboratory of Nuclear Problems at JINR.