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We elaborate on an earlier discussion of CP conservation of strong interactions which includes the effect of pseudoparticles. We discuss what happens in theories of the quantum-chromodynamics type when we include weak and electromagnetic interactions. We find that strong CP conservation remains a natural symmetry if the full Lagrangian possesses a chiral U (1) invariance. We illustrate our results by considering in detail a recent model of (weak) CP nonconservation.
Peccei et al. (1977) studied this question.