Flavor constraints for a Vector-like quark of Nelson-Barr type

TL;DR

This paper investigates flavor constraints on a specific type of vector-like quark in the Nelson-Barr model, showing that its parameter space is highly restricted, thus making it testable and falsifiable.

Contribution

It provides the first detailed flavor analysis of NB-type VLQs, demonstrating their constrained parameter space compared to generic VLQs.

Findings

Allowed parameter space is significantly smaller for NB-type VLQs.

The NB model's VLQ couplings are highly restricted by flavor data.

The results make the NB VLQ scenario falsifiable.

Abstract

The Nelson-Barr (NB) mechanism to solve the strong CP problem assumes CP conservation, arranges vanishing $\bar{\theta}$ at tree-level and requires vector-like quarks (VLQs) to transmit the CP breaking to the SM. We analyze the flavor constraints coming from the presence of one such down type VLQ of NB type by performing a global fit on the relevant flavor observables. A comparison is made to the case of one 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.