A conference on the “History of the Neutrino” was organized in Paris in September 2018, giving the opportunity to create this website dedicated to the rich history and the many facets of this fascinating particle.

Neutrinos play an exceptional role in particle or nuclear physics as well as in astrophysics. Postulated by Pauli in 1930, they were named by Fermi in 1933 and experimentally discovered by Reines and Cowan in 1956. A second family of neutrinos was discovered in 1962 and a third in 1975. The CERN collider LEP proved in 1989 that 3 types of interacting neutrinos are enough in the standard model of particle physics. They are named electron-neutrino (νe), muon-neutrino (νμ) and tau-neutrino (ντ), associated to the charged leptons electron, muon and tau. They have no charge, a very tiny mass and interact only weakly, so these elusive particles can cross large quantity of matter (like the Sun or the Earth) without interacting.

Emitted in huge numbers (about 1020 per second) in nuclear reactors, they are also artificially produced in man-made accelerators which deliver intense neutrino beams. But the main source of neutrinos is the Universe itself: the relic neutrinos from the Big Bang have been wandering for more than 13.6 billion years, with a density of 330 per cm3 everywhere.

Starting with the fusion of two protons, nuclear reactions in the core of the Sun produce about 2 1038 electron-neutrinos per second, which means 65 billions of neutrinos per second per cm2 on Earth. Supernova explosions emit about 1058 neutrinos in a few seconds and the central engines of active galactic nuclei produce them abundantly.

On Earth, many neutrinos are produced by the interaction of high energy cosmic rays in the upper atmosphere and are also emitted by radioactive elements in the crust and the mantle of the Earth. We are bathed in neutrinos which cross us continually and abundantly.

Witnesses of the core of the Sun, solar electron-neutrinos have been observed since 1968, but their number is significantly less than what is predicted by solar models built by astrophysicists. It took more than 30 years to solve the problem of the deficit, when the SNO experiment showed in 2001 that part of the solar electron-neutrinos had been transformed into mu-neutrinos or tau-neutrinos. This was explained by the fact that neutrinos were oscillating between the three families, a mechanism invented by Pontecorvo in 1958 and authorized by quantum mechanics (mechanism completed by the MSW effect for solar neutrinos). In fact the oscillation mechanism was first observed in 1998 by the SuperKamiokande experiment via the study of atmospheric neutrinos: muon-neutrinos produced in the atmosphere at the antipodes were oscillating into tau-neutrinos during their travel through the Earth. Neutrino oscillation is possible only if neutrinos have a mass (which is not necessary in the minimal standard model of particle physics) and its discovery opens the door towards, at least, the completion of the standard model.

Since 1970, neutrino beams have been used also to study neutrino properties but also to penetrate deep inside the nucleons and unveil their fine structure.

This website is dedicated to the various historical (experimental and theoretical) steps of particle physics linked to the use or to the knowledge of neutrino.

For most topics it is possible to post comments or corrections which after moderation will appear at the bottom of the article. General comments, suggestions, corrections are also welcome at the address:
michel.cribier (at) cea.fr

Special thanks to Jaime Houque Dawson, Stéphane Lavignac, Laurent Simard and Matthieu Vivier for their contributions. All our recognition to Grégoire Duval for his irreplaceable technical assistance.

Michel Cribier, Didier Verkindt, Daniel Vignaud

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