Strong CP and the PMNS Phase in Dirac Left-Right Symmetry

Abstract

Left--right symmetric theories with generalized parity restrict the bare QCD angle to a CP-conserving value and relate the physical strong-CP phase \(\barθ\) to a CP-odd parity-breaking parameter \(\eps\). We show that, in the Dirac-neutrino realization, imposing a sectorial reality condition on the Dirac lepton Yukawa matrices turns observable leptonic CP violation from an independent input into a branch-dependent prediction. Parity reconstructs the right-handed leptonic mixing matrix, whereas leptonic reality requires it to be rephasing-equivalent to the complex conjugate of the left-handed one. For generic three-generation Yukawas, compatibility is equivalent to the vanishing of a single Jarlskog-type CP-odd invariant. In the physical Dirac hierarchy, for fixed oscillation data, mass ordering, and discrete leptonic branch, compatibility determines the PMNS Jarlskog invariant as a function of the lightest neutrino mass and \(\eps\). We derive its leading analytic behavior and obtain the nonlinear compatibility branches numerically. In compressed mixed-sign Dirac-neutrino spectra, small signed mass sums can amplify a tiny parity deformation into order-one values of the normalized PMNS Jarlskog invariant, including maximal CP violation. The corresponding quark reconstruction independently provides a calculable, branch-dependent conversion between \(\eps\) and \(\barθ\). Together, the leptonic and quark relations define a family of correlations among leptonic CP violation, the absolute neutrino mass scale, and strong CP.