(© Kamioka Observatory, ICRR, University of Tokyo

Neutrinos, elementary particles that interact very little with matter, come in three types: e (the type of neutrinos emitted during beta decay), muon and tau (named after the heavy electrons which they are associated, the muon and tau). Theorists had predicted in 1956 that if neutrinos have nonzero masses, they can spontaneously transform from one species to another – a phenomenon called oscillation because the corresponding probabilities are periodic functions of time. The existence of neutrino oscillations has been confirmed for the first time in 1998 thanks to the Super-Kamiokande detector Japanese. The results of the international experience T2K, Japan, are now providing the last piece of the puzzle of neutrino oscillations.

In 1998, Super-Kamiokande was detected in the particles produced by interactions between cosmic rays and the upper atmosphere, fewer muon neutrinos than expected, we could interpret as a consequence of the transformation of some of muon neutrinos into tau neutrinos. Since then, transformation of electron neutrinos into muon neutrinos or tau have been demonstrated by various experiments on neutrinos emitted by the Sun or a nuclear reactor.

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