Symmetry properties of leptons in the zero-mass limit

Summary  The symmetry properties of all known leptons are studied for those processes in which their mass differences can be neglected. It is found that in this approximation both the electromagnetic interactionH elect and the weak interactionH weak are invariant under aU 2×U 2 group of transformations. Thus, in the zero-mass limit, all lepton systems are characterized by six quantum numbers (in addition to charge, momentum, etc.). The electromagnetic interaction is known to be further invariant under a discrete symmetry operatorC (charge conjugation), orP (parity). It is proposed that the weak interaction is also invariant under a discrete symmetry operator, calledD, which exchanges charged leptons and neutral leptons. TheC symmetry is violated byH elect, and theD symmetry is violated byH weak and by the mass differences. Applications of these symmetry properties to high-energy lepton reactions are discussed.