Leptons are 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.

Excerpt from the Encyclopedia Britannica without permission.