One Phase to Rule Them All: Spontaneous CP Violation and Leptogenesis in SO(10)

Abstract

We present a renormalizable $SO(10)$ grand unified theory with a minimal Yukawa sector consisting of a $126_H$, a real $10_H$ and a real $120_H$, where $CP$ violation has a spontaneous origin. We show that the Yukawa sector in this setup, which consists of only 19 real parameters, is capable of simultaneously reproducing the observed fermion masses and mixings, including neutrino oscillations, as well as the baryon asymmetry of the Universe via thermal leptogenesis. In this framework, $CP$ is spontaneously broken when a $CP$-odd Higgs field $54_H$, used for GUT symmetry breaking, acquires a non-zero vacuum expectation value. All $CP$-violating phases, including the Dirac phases $\delta_\mathrm{CKM}$ and $\delta_\mathrm{PMNS}$, the neutrino Majorana phases, as well as those responsible for leptogenesis, arise solely from a single complex parameter in the Higgs potential. The proposed minimal setup predicts a normal ordering of neutrino masses, with the atmospheric mixing angle $\theta_{23}$ preferred in the first octant and the leptonic $\delta_{\mathrm{PMNS}}$ lying in the range $(-38^\circ,\, +31^\circ)$. The fermion fits in our scenario further yield a strongly hierarchical mass spectrum for the three right-handed neutrinos, $(M_1,\,M_2,\,M_3) \sim \left(10^{5},\, 10^{12},\, 5\cdot 10^{14}\right)~\mathrm{GeV}$, which is shown to result in successful $N_2$-dominated leptogenesis, consistent with current cosmological data.