chargeconjugated

Charge conjugation is a symmetry operation in quantum field theory that transforms a particle into its antiparticle by reversing additive quantum numbers such as electric charge, baryon number, and lepton number, while leaving mass, momentum, and spin unchanged. The transformation does not alter the particle’s kinematic variables or its interaction with neutral fields, but it changes interactions that depend on charge.

In field theory, charge conjugation is implemented by an operator C that acts on fields. For a Dirac field ψ, the charge-conjugated field is ψ^C = C \bar{ψ}^T, where C is a matrix satisfying C γ^μ C^{-1} = - (γ^μ)^T. On particle states, C maps a particle to its antiparticle, up to a phase. Some neutral states are eigenstates of the C operator, possessing a well-defined C-parity with eigenvalues +1 or −1; examples include the neutral pion (π^0) and the photon, for which the C-parity is a useful quantum number in analyzing decays and production processes.

Charge conjugation plays a key role in discussions of CP and CPT symmetries and in studies of meson mixing and decay patterns. It is a meaningful concept for self-conjugate particles, where the state is identical to its antiparticle up to a phase, as well as for charged particles, where C relates distinct particle and antiparticle states (for example, electron and positron).

Limitations include that not all particles have a well-defined or conserved C-symmetry in all interactions; in particular, charged states do not form C-eigenstates in the same sense as neutral, self-conjugate states. See also antiparticle, CP symmetry, CPT symmetry.