A minimal model with natural suppresion of strong CP violation

Abstract

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 θ is zero in tree approximation. The one-loop contribution to θ 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 θ suppression. In particular, we argue that the one-loop correction to θ is further suppressed by the ratio 〈ø〉 2 〈S〉 2 . From the experimental limit on θ one derives the lower bound 〈 S〉 ⩾ 1 TeV, 100 TeV, for Yukawa couplings of order 10 −2, 10 −1 respectively.