Neutrinos are elementary particles that come in three "flavours"—electron, muon and tau. They are extremely light, carry no electric charge and rarely interact with other matter, streaming through the universe largely unimpeded. Hundreds of trillions pass through a human body every few seconds. They are produced by the nuclear reactions that fuel stars and atomic power plants.
The Standard Model of particle physics, one of the most successful scientific theories ever devised, predicts that neutrinos should have no mass at all. Around 30 years ago, however, scientists working at Super-Kamiokande, a neutrino observatory in Japan, noticed that fewer muon neutrinos were arriving from below (having travelled through the Earth) than from above, even though both sets were created by the same kind of cosmic-ray collisions in the atmosphere. Shortly afterwards the Sudbury Neutrino Observatory in Canada reported a similar anomaly with solar neutrinos: too few were electron-flavoured. These observations led physicists to conclude that neutrinos "oscillate"—transforming from one flavour to another as they travel through space—which is possible only if they possess some mass, however tiny. "Neutrino physics is physics beyond the Standard Model," as Juan Pedro Ochoa-Ricoux, a physicist at the University of California, Irvine, puts it.
Each neutrino flavour is a mixture of three underlying mass states, known as v1, v2 and v3. As a neutrino flies through space the exact combination of these states changes, causing the particle to switch flavour. Pinning down which mass state is heaviest and which lightest—known as the "mass ordering"—is one of the field's central goals. Current evidence leans towards "normal ordering", in which v1 is lighter than v2, both of which are much lighter than v3. The alternative, "inverted ordering", places v3 as the lightest.
JUNO (Jiangmen Underground Neutrino Observatory) is a Chinese facility located more than half a kilometre underground beneath Dashi Hill in Guangdong province. Ten years in the making, it began operations in August 2025. Its core is a 12-storey-high sphere of steel and plexiglass containing around 20,000 tonnes of hydrogen-rich liquid scintillator. When the rare neutrino strikes a proton in the fluid, a burst of blue light is detected by some 40,000 photomultiplier tubes lining the inside of the tank. The observatory sits 53km from a pair of nuclear power plants whose neutrino output is well characterised; scientists count how many neutrinos arrive, thereby measuring the rate at which oscillation occurs. About 700 metres of granite mountain shields the detector from cosmic rays. Around 50 neutrino detections are expected per day; roughly 100,000 will be needed for statistically significant results, a process the observatory's lead scientist, Wang Yifang (director of the Institute of High-Energy Physics at the Chinese Academy of Sciences), estimates will take about six years. JUNO will also study neutrinos from deep within the Earth and from supernovae. Because neutrinos flow through matter in ways light cannot, they can escape an exploding star before the blast becomes visible, giving astronomers advance warning to orient their telescopes.
Super-Kamiokande, in Japan, was the observatory where neutrino oscillation was first noticed. The Sudbury Neutrino Observatory, in Canada, independently confirmed the anomaly with solar neutrinos.
A group of extensions to the Standard Model, known as "seesaw" models, suggest that neutrinos may be their own antiparticles—a property that could explain their tiny masses by linking them to other, much heavier, as-yet-undetected neutrinos. Some theorists believe these heavier neutrinos could be candidates for dark matter. To test the hypothesis, physicists study radioactive isotopes of elements such as calcium and germanium, looking for a version of double-beta decay in which no antineutrinos are emitted. If the mass ordering is inverted, such events should occur often enough for sensitive experiments—including the LEGEND experiment in Italy and the NEXT experiment in Spain—to detect them within ten to 15 years. If the ordering is normal, the process would probably be too rare to observe with any detector that scientists know how to build.
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