LEPTON

in particle physics, any member of a class of fermions that respond only to electromagnetic, weak, and gravitational forces and do not take part in strong interactions. Like all fermions, leptons have a half-integral spin. (In quantum-mechanical terms, spin constitutes the property of intrinsic angular momentum.) Leptons obey the Pauli exclusion principle, which prohibits any two identical fermions in a given population from occupying the same quantum state. Leptons are said to be fundamental particles; that is, they do not appear to be made up of smaller units of matter. Leptons can either carry one unit of electric charge or be neutral. The charged leptons are the electrons, muons, and taus. Each of these types has a negative charge and a distinct mass. Electrons, the lightest leptons, have a mass only 0.0005 that of a proton. Muons are heavier, having more than 200 times as much mass as electrons. Taus, in turn, are approximately 3,700 times more massive than electrons. Each charged lepton has an associated neutral partner, or neutrino (i.e., electron-, muon-, and tau-neutrino), that has no electric charge and no significant mass. Moreover, all leptons, including the neutrinos, have antiparticles called antileptons. The mass of the antileptons is identical to that of the leptons, but all of the other properties are reversed. The total number of leptons appears to remain the same in every particle reaction. Mathematically, total lepton number L (the number of leptons minus the number of antileptons) is constant. In addition, a conservation law for leptons of each type seems to hold. The number of electrons and electron neutrinos, for example, is conserved separately from the number of muons and mu-neutrinos. The current limit of violation of this conservation law is one part per million. The electroweak theory of electromagnetic and weak interactions, proposed during the late 1960s, has enabled physicists to better understand the interactions of leptons. This apparent theoretical conquest, however, has also generated a host of new questions. Other, more recent theoretical schemes seeking to intertwine strong interactions with the weak and the electromagnetic have had a similar effect. A law akin to that of the conservation of lepton number exists for strongly interacting fermions, the baryons (e.g., protons). The new grand unified theories suggest that a proton decays into leptons and other particles, thereby simultaneously violating lepton and baryon number conservation. In such theories the quantity B - L, the number of baryons minus the number of leptons, is conserved. See also fermion; gauge theory.