C parity

In physics, the C parity or charge parity is a multiplicative quantum number of some particles that describes their behavior under the symmetry operation of charge conjugation.

Charge conjugation changes the sign of all quantum charges (that is, additive quantum numbers), including the electrical charge, baryon number and lepton number, and the flavor charges strangeness, charm, bottomness, topness and Isospin (I₃). In contrast, it doesn't affect the mass, linear momentum or spin of a particle.

Consider an operation ${\mathcal {C}}$ ${\mathcal {C}}$ that transforms a particle into its antiparticle,

Both states must be normalizable, so that

which implies that ${\mathcal {C}}$ ${\mathcal {C}}$ is unitary,