Strong CP violation: Problem or blessing?

Abstract

We readdress the issue of strong CP violation both in the Standard Model (SM) and in the Minimal Left–Right (LR) Symmetric Model and try to clear the confusion that seems to still pervade the field. We argue that the smallness of strong CP violation, while harmless and basically decoupled from the rest of the physics in the SM, in the context of LR symmetry provides a blessing by helping to narrow down the parameter space of the theory and connecting apparently the uncorrelated physical quantities. In particular, in the context of left–right symmetry being parity, it either points to the suppression of leptonic CP violation noticed before, or leads to relatively light right-handed neutrinos, potentially accessible at the next hadron collider. The latter, more natural in view of complex quark Yukawa couplings, goes hand in hand with the smallness of lepton flavor violation and enhances the possibility of observing neutrinoless double-beta decay.