Quantum states of a particle and an antiparticle are interchanged by the combined application of charge conjugation C , parity P and time reversal T . C and P are linear, unitary operators, T is antilinear and antiunitary, ⟨ ⟨ Ψ Ψ | T Φ Φ ⟩ ⟩ = ⟨ ⟨ Φ Φ | T − − 1 Ψ Ψ ⟩ ⟩ . If | p , σ σ , n ⟩ ⟩ denotes the quantum state of a particle n with momentum p and spin J whose component in the z-direction is σ σ , then one has