Neutrino oscillation

Neutrinos are electrically neutral particles that interact through the weak force. In experiments they are identified by flavour: electron, muon or tau, associated with the charged lepton produced in an interaction. The states with definite flavour are not the same as states with definite mass. Instead, each flavour state is a quantum-mechanical combination of neutrino mass states. As these components travel, their phases evolve at different rates. When they are later detected together, the probability of finding a given flavour depends on the distance travelled and the neutrino energy. This changing probability is called oscillation; it does not mean that a neutrino periodically changes its mass or electric charge.

Experiments can look for fewer neutrinos of one flavour than expected, or for the appearance of a different flavour. Super-Kamiokande studied atmospheric neutrinos produced when cosmic rays interact with the atmosphere. Its observations showed that the number of upward-travelling muon neutrinos fell relative to downward-travelling ones, consistent with the longer journey through Earth and conversion largely into tau neutrinos. The Sudbury Neutrino Observatory (SNO) examined neutrinos from the Sun using heavy water. Its measurements were sensitive both to electron neutrinos and to the combined flux of all three flavours; the comparison showed that solar electron neutrinos had converted into other flavours. These complementary measurements helped distinguish flavour conversion from an explanation based solely on a lower neutrino production rate in the Sun.

Oscillation requires differences between neutrino mass states: if all relevant states were massless or had identical masses, their relative phases would not develop in the way needed for flavour change. Thus, observation of oscillations establishes that at least two neutrino mass states have non-zero mass. The minimal Standard Model treats neutrinos as massless, so this result demonstrates that the model needs extension. Oscillation experiments measure mass-squared differences and mixing parameters, rather than directly determining the absolute mass of each neutrino. Questions about the origin and ordering of neutrino masses remain subjects of research.

The possibility of neutrino flavour change was proposed decades before it was observed. In 1998, the Super-Kamiokande collaboration presented compelling evidence from atmospheric muon-neutrinos. In 2001 and 2002, SNO published results showing conversion of solar electron-neutrinos into other flavours. These discoveries established oscillation as an important part of modern particle physics and astrophysics.