Nonaxion solution of the problem of CP conservation in strong interactions
Krasnikov N. V., Matveev V. A.
A new model based on the gauge group SU(9) $ SU(2) ® U(l) furnishes a natural explanation of why the CP nonconservation is slight in strong interactions, without hypothesizing the existence of an axion. Supersymmetry models can also explain the absence of strong CP violation.