Text: A fermion having a mass smaller than the proton mass; leptons interact with electromagnetic and gravitational fields, but beyond this they interact only through weak interactions. One prominent representative of the lepton species, which comes in three families, each with an electrically charged lepton and a neutral neutrino, is the electron. It is known to play the all-important role in chemical and electrical phenomenoa. According to present-day knowledge, the leptons are elementary, i.,. are indivisible and have no substructure. The proton (and neutron), i.e., nuclear matter, on the other hand, has been shown to be composed of more elementary units, the quarks. Supposedly six different kinds of quarks exist, again grouped in three families, in complete symmetry to the leptons. Five of these quarks have been found, and recent experimental evidence exists for the sixth quark. Only the two members of the first family are needed for "ordinary" nuclear matter, the "u" and the "d" quark. However, the quarks seem to always be bound together to form the nuclear particles, called hadrons, and never have been observed as free particles: Three such quarks make up the proton, two u quarks and one d quark, while the neutron consists of one u quark and two d quarks. Hadrons containing the heavier quarks, s, c, and b have also been observed. These particles, however, have very short lifetimes and are being produced under "natural" condition[s] mostly in reactions of cosmic rays with nuclei in the terrestrial atmosphere.

See Also: ELECTRON; PROTON; NEUTRON; HADRON; QUARK; ATOMOLINI; ATOMOLE

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