Variant Nelson-Barr Mechanism with Minimal Flavor Violation

TL;DR

This paper proposes a variant Nelson-Barr model with minimal flavor violation that cancels the strong CP phase using heavy fermions, predicts numerous Goldstone bosons, and explores their astrophysical and cosmological implications, including contributions to $ _{eff}$.

Contribution

It introduces a novel Nelson-Barr model with minimal flavor violation, suppressing corrections to the strong CP phase and analyzing the cosmological effects of Goldstone bosons.

Findings

Suppression of higher-order corrections to the strong CP phase below experimental limits.

Prediction of multiple Goldstone bosons affecting astrophysical constraints.

A new 'flavor stairway' pattern in $ _{eff}$ related to the SM quark flavor structure.

Abstract

Within the general framework of using spontaneous CP violation to solve the strong CP problem, we construct a variant Nelson-Barr model in which the Standard Model (SM) quark contribution to the strong CP phase is cancelled by new heavy QCD-charged fermions. This cancellation is ensured by choosing conjugate representations for the new colored states under the same global flavor symmetry of SM quarks. Choosing the global flavor symmetry to be that of minimal flavor violation, we suppress higher-order corrections to the strong CP phase to well below current experimental constraints. More than two dozen massless Goldstone bosons emerge from spontaneous flavor symmetry breaking, which yield strong astrophysical constraints on the symmetry breaking scale. In the early universe, the Goldstone bosons can be thermally produced from their interactions with the heavy colored fermions and contribute to $\Delta N_{\rm eff}$ at a measurable level. As a function of reheating temperature, the predicted $\Delta N_{\rm eff}$ shows an interesting plateau behavior we dub the ``flavor stairway", which encodes information about the SM quark flavor structure.