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Any member of a class of fermions that respond only to electromagnetic, weak, and gravitational forces and do not take part in strong interactions. Leptons have a half-integral spin and obey the Pauli exclusion principle. They can either carry one unit of electric charge or be neutral. The charged leptons are the electrons, muons, and taus. Each has a negative charge and a distinct mass. Each charged lepton has an associated neutral partner, or neutrino, which has no electric charge and very little if any mass.
This entry comes from Encyclopædia Britannica Concise. For the full entry on lepton, visit Britannica.com.