Nelson-Barr solutions to the strong CP problem

Tuesday, 14 November 2023 at 11:15

The Nelson-Barr mechanism provides a solution to the strong CP problem where CP is spontaneously broken in extensions of the standard model that involve new colored fermionic degrees of freedom. The simplest implementation of such a mechanism consists of the simplified model proposed by Bento, Branco, and Parada in 1991, where a pair of vector-like down quarks, a scalar field and a Z2 symmetry are assumed. In Nelson-Barr models, the tree-level contribution to the QCD vacuum angle (the basis invariant parameter that enters in the neutron electric dipole moment) is predicted to be zero consistently with the generation of an order one phase in the CKM matrix. In this talk we will review the implementation of such a mechanism and will derive explicit expressions for the flavor changing couplings of the electroweak and Higgs bosons. We will present the simplest UV completion, based on Quark-Lepton unification, where the mechanism is implemented automatically, and will discuss interesting phenomenological consequences associated to it (ongoing work).