A MINIMAL MODEL WITH NATURAL SUPPRESSION OF STRONG CP VIOLATION

We suggest a simple solution to the strong CP problem, within a minimal extension of the standard model, where the only additional fields are a charge -1/3 vector-like quark and a singlet complex scalar S. The model satisfies the criteria of Barr and Nelson and thus the parameter THETABAR is zero in tree approximation. The one-loop contribution to THETABAR is suppressed by Yukawa couplings, a general feature of this class of models. Furthermore, we show that in the proposed model there is a close connection between the hierarchy problem and the mechanism for THETABAR suppression. In particular, we argue that the one-loop correction to THETABAR is further suppressed by the ratio <phi>2/<S>2. From the experimental limit on THETABAR one derives the lower bound <S> greater-than-or-equal-to TeV, 100 TeV, for Yukawa couplings of order 10(-2), 10(-1) respectively.