A Chiral Nelson-Barr Relaxion

Abstract

We present the chiral Nelson-Barr relaxion model, an extension of the Nelson-Barr relaxion framework with a gauged chiral $\mathrm{U}(1)$ symmetry. It includes vector-like quarks to cancel gauge anomalies and a scalar whose vacuum expectation value generates their masses. The chiral symmetry forbids the leading operators responsible for the Nelson-Barr and clockwork quality problems. Right-handed neutrinos generate light-neutrino masses through the type-I seesaw mechanism and permit thermal leptogenesis. For the particular Nelson-Barr sector Yukawa coupling considered, relaxion phenomenology gives a maximal scale of $CP$ violation $f_{\text{max}}^{\text{pheno}}\sim 4\times 10^{11}\mathrm{GeV}$, about an order of magnitude greater than the corresponding phenomenological ceiling in the original model. The model can accommodate thermal leptogenesis if its right-handed-neutrino masses and reheating history satisfy the Davidson-Ibarra bound.

arXiv comment34 pages, 5 figures, 2 tables