A Grand-Unified Nelson-Barr Model

TL;DR

This paper presents a grand unified model based on E6 that naturally incorporates the Nelson-Barr solution to the Strong CP Problem, predicts fermion observables, and links neutrino CP violation to the CKM phase.

Contribution

It develops a novel E6 GUT framework implementing Nelson-Barr mechanism, providing predictive power for fermion masses and CP phases, and connecting neutrino and quark CP violation.

Findings

Successful fit to all SM observables

Predicted neutrino Dirac CP phase correlated with CKM phase

Testable predictions for neutrino CP violation in future experiments

Abstract

We argue that the Nelson-Barr solution to the Strong CP Problem can be naturally realized in an E$_6$ Grand-Unified Theory. The chiral SM fermions reside in three generations of E$_6$ fundamentals together with heavy vectorlike down quarks, leptons doublets and right-handed neutrinos. CP is imposed on the Lagrangian and broken only spontaneously at high scales, leading to a mixing between chiral and vectorlike fields that allows to solve the Strong CP Problem through the Nelson-Barr mechanism. The main benefit of the E$_6$ GUT structure is the predictivity in the SM fermion sector, and a perfect fit to all SM observables can be obtained despite being over-constrained. Definite predictions are made for the neutrino sector, with a Dirac CP phase that is correlated to the CKM phase, allowing to test this model in the near future.