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Linné on line arrow Physics and the Cosmos arrow Physics and the Cosmos arrow Quarks and leptons arrow Why are there three families of elementary particles?

Why are there three families of elementary particles?

Apart from the electron there are two other electrically charged leptons: the muon which was discovered in cosmic rays in the 1930's and the tau-lepton which was discovered in 1975 by Martin Perl and his collaborators (for this he shared the Nobel Prize in Physics in 1995). The muon and the tau-lepton have the same properties as the electron except that they are much heavier. The muon weighs about 200 times more than the electron whereas the tau-lepton is about 4000 times as heavy. For each electrically charged lepton there is also one so called neutrino which is electrically neutral. For a long time it was thought that neutrinos are massless, but today (2003) we know they are actually massive. We still do not know the actual masses, only that they are very small compared to the electron mass. All in all we know of three different types of lepton pairs. In the same way there are also three different quark-pairs. To the question why there are three families of elementary particles we do not have a fundamental answer today. We only know that there are no more than three types of neutrinos with masses smaller than half the Z-boson mass.