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In the 1920s, physicists noticed some discrepancies in beta decay experiments. In beta decay, a neutron decays into a proton by emitting an electron, also termed a beta particle. It was observed that the total momentum and energy of the electron and proton after the decay was sometimes less than the initial momentum and energy of the neutron. Where did the missing momentum and energy go? According to fundamental laws of physics, the total amounts of both momentum and energy must remain constant. Neither can just disappear.

In 1930, the Austrian physicist Wolfgang Pauli (1900–1958) wrote a letter to a gathering of physicists in Tübingen, Germany, in which he suggested the idea of neutrinos as the particles that carry away the missing energy. However, he did not publish the idea for another three years. Pauli originally called his suggested particle the neutron, as neutrons had not been discovered in 1930. When neutrons were discovered, the term "neutron" was taken, so Pauli's particle became the neutrino: literally, the little neutral one.

Italian physicist Enrico Fermi's (1901–1954) 1934 theory of beta decay used the neutrino hypothesis. (This theory, still used for approximate calculations, was only surpassed for more accurate calculations by theories developed in the 1970s.) But did neutrinos really exist? In the 1930s, no experiments to detect them were possible.

In 1956, after a four-year search, U.S. physicists F. Reines (1918–1998) and C. L. Cowan (1919–1974) finally succeeded in detecting neutrinos produced by the Savannah River Reactor in South Carolina. By 1962, a particle accelerator at Brookhaven National Laboratory was generating enough neutrinos to conduct detection experiments. Over several months, physicists observed a few dozen neutrino events and found that there were at least two types of neutrinos. The first one discovered was dubbed the electron neutrino, and the second the muon neutrino. Proof of a long-suspected third type of neutrino, the tau neutrino, were first found in late 1998.

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