Strong $\mathrm{CP}$ invariance is automatically preserved by a spontaneously broken chiral ${\mathrm{U}\left(1\right)\mathrm{}}_4$ symmetry. A weak-interaction singlet heavy quark $Q$, a new scalar meson ${\sigma }^0$, and a very light axion are predicted. Phenomenological implications are also included.

• Received 16 February 1979

DOI:https://doi.org/10.1103/PhysRevLett.43.103