JUNO

Background. By the 2010s the broad pattern of neutrino oscillation was firmly established, yet one stubborn unknown remained: the neutrino mass ordering. Reactor experiments such as Daya Bay had measured the small mixing angle theta-13, opening a path to resolve the ordering with reactor neutrinos alone, but only with an instrument of exceptional energy resolution.

Objective. Determine the neutrino mass ordering and measure several oscillation parameters to sub-percent precision, sharpening the global picture of how the three neutrino states mix.

Method. At its heart sits a 20-kilotonne acrylic sphere of liquid scintillator, buried about 700 m underground and positioned roughly 53 km from a cluster of nuclear reactors. That baseline is chosen deliberately. It is where the slow, solar-driven oscillation and the faster, atmospheric-driven oscillation overlap, imprinting a delicate interference pattern on the antineutrino energy spectrum. Reading that pattern demands the best energy resolution ever attempted at this scale, achieved with tens of thousands of high-coverage photomultiplier tubes.

Results. Data-taking began in August 2025. Within weeks the detector measured the solar oscillation parameters theta-12 and the solar mass splitting more precisely than all previous experiments combined, an early demonstration that the hardware performs as designed. The mass-ordering determination is a slower harvest and will sharpen as exposure accumulates over the coming years.

Significance. JUNO settles the ordering through a vacuum interference effect that does not rely on matter effects in the Earth, making it cleanly complementary to long-baseline beam experiments. The same detector also serves solar, supernova, and geoneutrino physics, so even before the ordering is fixed it is already a versatile observatory. Its precision parameters will feed directly into the broader program of oscillation measurements now underway worldwide.