After 17 years of planning and construction, the Jiangmen Underground Neutrino Observatory (JUNO) in China has officially opened. Just two months into operation, this impressive detector has already produced astonishing results in the field of particle physics.
At 20,000 tons, JUNO is a giant spherical detector located 700 meters underground in Guangdong province. It’s designed to study neutrinos—tiny particles that pass through us by the trillions every second, yet leave no mark. With its unmatched size and sensitivity, JUNO aims to solve a long-standing mystery: the mass ordering of neutrinos.
Inside this cutting-edge facility is a 35.4-meter acrylic sphere filled with liquid scintillator, which lights up when neutrinos interact with it. Surrounding this core are 43,212 photomultiplier tubes that can detect even a single photon. To further shield the detector from cosmic rays and noise, a large water Cherenkov detector and a plastic scintillator lay protective layers around it.
JUNO is strategically placed 52.5 km from the Yangjiang and Taishan nuclear power plants. This distance is optimal for detecting reactor antineutrinos, which are the primary focus of its research. Since it began collecting data on August 26, 2025, JUNO has achieved a remarkable 97.8% operational efficiency during early testing.
The main goal of JUNO is to determine the neutrino mass hierarchy—the order of masses among the three types of neutrinos: electron, muon, and tau. Resolving this is a crucial question in physics. While we know these particles have different masses, their exact order continues to puzzle scientists. JUNO plans to clarify this using precise measurements of how neutrinos oscillate.
At a recent press conference, spokesperson Yifang Wang noted that in just 59 days, JUNO has achieved a measurement precision 1.8 times greater than previous experiments for solar neutrino oscillations. The detector confirmed a previously noted discrepancy in measurements between solar and reactor neutrinos. “With this accuracy, JUNO is poised to clarify the neutrino mass order and explore concepts beyond the three-flavor oscillation model,” Wang stated.
JUNO’s impressive results are not only due to its size; extreme precision and purity are equally crucial. The liquid scintillator underwent five purification processes to achieve an attenuation length of 20.6 meters, exceeding expectations for optical clarity. All internal surfaces were meticulously cleaned, and equipment operated under continuous nitrogen flow to avoid radon contamination. The levels of uranium and thorium found in the liquid are significantly below what was planned.
A sophisticated calibration network ensures that every photon detection is accurately tracked back to its source. According to a report published in *Chinese Physics C*, the energy resolution for detecting specific gamma rays achieved an impressive 3.4%, confirming the detector’s high performance for its size.
JUNO stands prepared for decades of discovery, reflecting extensive international collaboration. The project brings together over 700 scientists from 75 institutions across 17 countries. From initial design to the logistics of filling the detector with ultra-pure materials, unwavering dedication has been essential.
Additionally, JUNO shows promise in monitoring various neutrino types, including those from solar activity, atmospheric events, and even potential supernovae. As physics analysis coordinator WEN Liangjian noted, the convergence of expertise in liquid scintillator detectors has been pivotal in achieving JUNO’s outstanding capabilities.
Looking ahead, the scientific community eagerly anticipates the insights that will materialize from JUNO’s findings. This groundbreaking facility is set to transform our understanding of the universe and the fundamental particles that make it up.
