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Neutrino Oscillations Explained
Neutrinos are nearly massless particles that barely interact with matter: trillions pass through you every second. They come in three flavours: electron, muon and tau. Remarkably, a neutrino born as one flavour can later be detected as another. This is neutrino oscillation.
Why neutrinos change flavour
Each flavour is a quantum mixture of three mass states. Because the mass states have slightly different masses, they travel with slightly different phases. As the neutrino moves, the mixture drifts, and the chance of finding each flavour rises and falls.
The oscillation formula
In the simplest two-flavour picture, the probability of a flavour change after distance L is P = sin²(2θ) · sin²(1.27 Δm² L / E), with Δm² in eV², L in km and E in GeV. θ is the mixing angle and Δm² the difference of the squared masses. So oscillations need mixing and unequal masses.
Why it matters
Oscillations prove that neutrinos have mass, something the original Standard Model of particle physics did not include. Takaaki Kajita and Arthur McDonald won the 2015 Nobel Prize for discovering them, with the Super-Kamiokande and SNO experiments.
Experiments today
Long-baseline experiments such as T2K (Japan), NOvA (USA) and the upcoming DUNE fire neutrino beams hundreds of kilometres through the Earth to measure the mixing precisely and to test whether neutrinos and antineutrinos behave differently.
Frequently asked questions
What are neutrino oscillations?
The changing of a neutrino from one flavour (electron, muon or tau) to another as it travels, because each flavour is a mixture of mass states with slightly different masses.
Do neutrinos have mass?
Yes. Oscillations would be impossible if all neutrinos were massless. Their masses are tiny, less than a millionth of the electron's.
Who discovered neutrino oscillations?
The Super-Kamiokande experiment (Japan) and the Sudbury Neutrino Observatory (Canada). Takaaki Kajita and Arthur McDonald received the 2015 Nobel Prize.