Soft Superweak CP Violation and the Strong CP Puzzle
  We discuss a class of models in which CP is violated softly in a heavy sector adjoined to the standard model. Heavy-sector loops produce the observed CP violation in kaon physics, yielding a tiny and probably undetectable value for $\epsilon^\prime$. All other CP-violating parameters in the effective low-energy standard model, including the area of the unitarity triangle and $\bar\theta$, are finite, calculable and can be made very small. The leading contribution to $\bar\theta$ comes from a four-loop graph. These models offer a natural realization of superweak CP violation and can resolve the strong CP puzzle. In one realization of this idea, CP is violated in the mass matrix of heavy majorana neutrinos. 
Howard Georgi and Sheldon L. Glashow