A new milestone: JUNO announced its first results
The Jiangmen Underground Neutrino Observatory (JUNO), using the first 59.1 days of data collected since August 26, 2025, announced its first results on reactor antineutrino oscillations on November 19, 2025, at a press conference held in Jiangmen.
The neutrino, one type of elementary particle, truly lives up to its name, “neutral (neutral) small (ino) particle.” It is nearly massless and notoriously difficult to catch, yet it permeates our surroundings. Wolfgang Pauli first postulated its existence in 1930, but the particle was not directly observed until results from an experiment by Clyde Cowan and Frederick Reines were reported in 1956. Thanks to large-scale neutrino detectors in the US, Japan, Canada, China, and other countries, physicists have gained substantial insight into how the three types of neutrinos—electron, muon, and tau—transform into each other, a phenomenon called neutrino oscillation. This behavior has been observed with neutrinos from many sources, including the Sun, the atmosphere, accelerators, and nuclear reactors.
In March 2012, the Daya Bay reactor neutrino experiment, the largest China-US basic research collaboration to date, precisely measured the third neutrino mixing angle θ13 and found it to be clearly non-zero with very high significance, with a value of sin22θ13 = 0.092 ± 0.016(stat) ± 0.005(syst).2 This completed the determination of all neutrino mixing angles and laid the foundation for later studies on neutrino mass ordering and charge conjugation and parity (CP) violation. Over the next 8 years, the team improved the precision of sin22θ13 6-fold to 2.8%, a milestone expected to remain unsurpassed for a few decades. The Daya Bay experiment ceased operations in December 2020 after successfully fulfilling all of its scientific missions.
In 2008, while the Daya Bay experiment was still under construction, Liang Zhan, Yifang Wang, Jun Cao, and Liangjian Wen had already begun planning China’s next step in neutrino research by utilizing reactor antineutrinos to determine the neutrino mass ordering.3 Although θ13 was still unknown at that time, they had initiated preliminary investigations into photomultiplier tube (PMT) technology and experimental site selection. Initially, they intended to continue using the Daya Bay Nuclear Power Plant (NPP) as the source of neutrinos. However, due to the NPP development strategy, it could not guarantee that the experimental site would be equidistant from all reactors, which might cause partial cancellation of oscillation signals from different reactors. By 2012, when the project officially advanced, Dashi Hill in Kaiping, near Jiangmen City in the Guangdong Province, was ultimately selected as the ideal site. It is equidistant from eight nuclear reactors in the Yangjiang and Taishan NPPs (at 52.5 km), and the project was subsequently renamed JUNO.
