A grand-unified Nelson–Barr model

Abstract We argue that the Nelson–Barr solution to the Strong CP Problem can be naturally realized in an E$$_6$$ 6 grand-unified theory. The chiral SM fermions reside in three generations of E$$_6$$ 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$$_6$$ 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.

Abstract licence: CC BY

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