When LEP started in 1989 the measurment of the mass width for Z had highest priority. During some hectic weeks, the probability to produce Z particles as a function of the energy of the colliding particles was measured. From the mass width of the Z particle the number of decay channels and thereby the number of families of quarks and leptons could be decided.

The news spread quickly and after a week it could be read in the newspapers that the fundamental building blocks in nature are six quarks and six leptons (plus their antiparticles), and after yet another week it could be read in scientific journals that the number of light neutrinos and thereby the number of families was 3.1±0.2. There are no more quarks nor leptons than the ones we know of now. The number of neutrinos is measured to be 2.99±0.01. The reason that the number could be any number (and not an integer) is that there could be other, unknown particles giving a smaller contribution than a neutrino.

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