Neutrinos are massless.

Neutrinos are extremely light particles produced in processes such as nuclear reactions in the Sun and radioactive decay.

They come in three types, or flavors: electron, muon, and tau neutrinos.

For decades, the simplest version of the Standard Model treated all neutrinos as exactly massless. That seemed reasonable because experiments could place only very small upper limits on their masses.

Then detectors found a persistent puzzle. They saw fewer neutrinos of particular flavors from the Sun and from cosmic-ray interactions in Earth’s atmosphere than calculations predicted.

One explanation was that the neutrinos were not disappearing at all. They were changing flavor while traveling.

Super-Kamiokande provided decisive evidence in 1998. Atmospheric muon neutrinos were depleted in a way that depended on the distance they had traveled through Earth. The Sudbury Neutrino Observatory later showed that the Sun was producing the expected total number of neutrinos, but many electron neutrinos had arrived as other flavors.

This behavior is called neutrino oscillation. In quantum mechanics, flavor states are mixtures of underlying mass states. For the flavor mixture to change as the particles travel, those mass states must have different masses.

The experiments therefore show that at least some neutrino masses are nonzero.

They do not yet tell us the complete absolute mass scale. Oscillation experiments measure differences between neutrino masses, while other experiments and cosmology continue to constrain how heavy the particles are overall.