How do neutrinos oscillate between flavor states?

Neutrinos oscillate because the states produced and detected by the weak interaction—electron, muon and tau neutrinos—are not the same as the states with definite mass. When a neutrino is created in a beta decay or in the core of the Sun, it emerges in a flavor state that is a quantum superposition of mass eigenstates. These mass eigenstates propagate with slightly different energies and hence accumulate different quantum phases as they travel. The changing relative phases alter the superposition, so the probability of observing a particular flavor changes with distance and energy. This quantum-mechanical interference is the essence of neutrino oscillation.

Quantum-mechanical mixing

The mathematical description uses a unitary mixing matrix that relates flavor eigenstates to mass eigenstates. Early theoretical groundwork was laid by Bruno Pontecorvo at the Joint Institute for Nuclear Research, who proposed the possibility of neutrino–antineutrino transitions and later flavor oscillations. The formal mixing framework was extended by later theorists to describe three flavors. Because the mass eigenstates have distinct masses, their different phase evolution produces oscillatory transition probabilities that depend on travel distance, energy, the differences of the squared masses, and the parameters of the mixing matrix. In matter, interactions with electrons modify these phases through the Mikheyev–Smirnov–Wolfenstein effect, which can enhance or suppress flavor conversion under certain densities and energies.

Experimental confirmation and significance

Experimental confirmation emerged from large underground detectors built to study neutrinos from the atmosphere, the Sun and reactors. The Super-Kamiokande collaboration led by Takaaki Kajita at the University of Tokyo provided clear evidence that muon neutrinos produced by cosmic-ray interactions change flavor as they cross the Earth. The Sudbury Neutrino Observatory experiment associated with Arthur B. McDonald at Queen's University in Canada demonstrated that solar electron neutrinos convert into other flavors before reaching the detector, resolving the long-standing solar neutrino deficit. These observations established that neutrinos have mass and that the Standard Model requires extension to accommodate mixing and mass terms.

Causes, consequences and wider context

The direct cause of oscillation is nonzero neutrino mass and nontrivial mixing; the deeper origin of the mass pattern remains an open question tied to physics beyond the Standard Model. Consequences are broad: neutrino masses influence cosmology through effects on structure formation and the cosmic neutrino background; oscillations enable neutrino flavor imaging of the Sun, supernovae and Earth's interior; and the possibility of CP violation in the neutrino sector could contribute to the matter–antimatter asymmetry of the universe. The construction and operation of massive detectors have involved communities near Kamioka in Japan and the mining town of Sudbury in Canada, where deep underground sites provide shielding from cosmic rays and reflect a cultural and territorial partnership between scientists and local industries. Continued global collaborations seek to measure remaining unknowns in the mixing matrix and mass ordering, linking precise laboratory work to questions about the origin and evolution of nature at the largest scales.