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The electron neutrino () is an elementary particle which has zero electric charge and a spin of . Together with the electron, it forms the first generation of , hence the name electron neutrino. It was first hypothesized by Wolfgang Pauli in 1930, to account for missing momentum and missing energy in beta decay, and was discovered in 1956 by a team led by Clyde Cowan and Frederick Reines (see Cowan–Reines neutrino experiment).
In 1930, Wolfgang Pauli theorized that an undetected particle was carrying away the observed difference between the energy, momentum, and angular momentum of the initial and final particles.Niels Bohr was notably opposed to this interpretation of beta decay and was ready to accept that energy, momentum, and angular momentum were not conserved quantities.
Dear radioactive ladies and gentlemen,As the bearer of these lines ... will explain more exactly, considering the 'false' statistics of N-14 and Li-6 nuclei, as well as the continuous β-spectrum, I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the energy theorem. Namely there the possibility that there could exist in the nuclei electrically neutral particles that I wish to call neutrons,Pauli means what was later named "neutrino". See § Name, above. which have spin and obey the exclusion principle, and additionally differ from photon in that they do not travel with the velocity of light: The mass of the neutron must be of the same order of magnitude as the electron mass and, in any case, not larger than 0.01 proton mass. The continuous β-spectrum would then become understandable by the assumption that in β decay a neutron is emitted together with the electron, in such a way that the sum of the energies of neutron and electron is constant.
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But I don't feel secure enough to publish anything about this idea, so I first turn confidently to you, dear radioactives, with a question as to the situation concerning experimental proof of such a neutron, if it has something like about 10 times the penetrating capacity of a gamma ray.
I admit that my remedy may appear to have a small a priori probability because neutrons, if they exist, would probably have long ago been seen. However, only those who wager can win, and the seriousness of the situation of the continuous β-spectrum can be made clear by the saying of my honored predecessor in office, Peter Debye, ... " One does best not to think about that at all, like the new taxes." ... So, dear radioactives, put it to test and set it right. ...
- With many greetings to you, also to Mr. Back,
- Your devoted servant,
- W. Pauli
A translated reprint of the full letter can be found in the September 1978 issue of Physics Today.
Upon the prediction and discovery of a second neutrino, it became important to distinguish between different types of neutrinos. Pauli's neutrino is now identified as the electron neutrino, while the second neutrino is identified as the muon neutrino.
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