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Up to this point, the leptons that were known included the electron, the electron neutrino, and their antiparticles and the muon, the muon neutrino, and their antiparticles. That amounted to eight leptons altogether.

In 1974 the American physicist Martin L. Perl found that when electrons and positrons (antielectrons) were smashed together at high energies, still a third variety of lepton was produced. This was named the tau electron, or in shorter version, the tauon. It presumably has a neutrino of its own, and there is surely an antiparticle for each, so that twelve leptons are now known.

The tauon is about 17 times as massive as the muon and about 3,500 times as massive as the electron. It is very unstable and lasts less than five-trillionths of a second before breaking down to a muon.

It may be that twelve leptons are all the leptons there are.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
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