Weak-Interaction Singlet and Strong<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>CP</mml:mi></mml:math>Invariance

Strong $\mathrm{CP}$ invariance is automatically preserved by a spontaneously broken chiral ${\mathrm{U}(1)}_{4}$ symmetry. A weak-interaction singlet heavy quark $Q$, a new scalar meson ${\ensuremath{\sigma}}^{0}$, and a very light axion are predicted. Phenomenological implications are also included.