The Nelson-Barr (NB) mechanism to solve the strong CP problem assumes CP conservation, arranges the theta angle to vanish at tree-level and requires vector-like quarks (VLQs) to transmit the CP breaking to the SM. We study the scenario where only these VLQs are within reach at the TeV scale while the spontaneous CP breaking sector is inaccessible. We investigate how these VLQs of Nelson-Barr type differ from generic VLQs and find from parameter counting that one less parameter is needed. In particular, for one VLQ of Nelson-Barr type, there is only one CP odd quantity that is responsible for all CP violation. In this case, we solve the technical problem of parametrizing only the new physics parameters while keeping the SM parameters as independent inputs. For one down-type VLQ, the model is largely flavor safe because the VLQ couplings to the SM up quarks and the W are hierarchically smaller for lighter quarks. We also analyze the flavor constraints in detail by performing a global fit on relevant flavor observables and compare the case of one VLQ of Nelson-Barr type with a generic VLQ. We find that the allowed parameter space for the VLQ Yukawa couplings and the mixing to the SM are confined to a region much smaller than in the generic case, making the NB case falsifiable in principle.
[The seminar will be recorded]
With support from FCT through project UIDB/00777/2020