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So far then, a charge 2/3 quark, a charge −1/3 quark, a neutral lepton, and a charge −1 lepton have been discussed. The masses of these particles in units of 109 electron volts (GeV) are shown in the first part of Table 2.1; and they comprise what is called a particle generation or family. A major surprise has been production in the laboratory of extra particle generations. A remarkable feature of nature that has been discovered is that this pattern of particles—two quarks and two leptons of the indicated charges—is repeated and then repeated again. Except for the neutrinos, which perhaps remain massless, the particles of each subsequent generation become heavier, as Table 2.1 shows. These additional generations appear to have nothing to do with "ordinary tangible matter." Yet they were important in the first moments of the universe and have a profound role in our understanding of nature.
The masses of the quarks and leptons range from zero, or near zero, for neutrinos to almost 200 times the proton mass for a t quark. Understanding why quarks and leptons exhibit this not quite random progression of masses is an important topic of research in elementary-particle physics.
Good experimental evidence exists that there are only three generations. Why this should be so constitutes a major mystery in the field.