Implications of the zero 1-3 flavour mixing hypothesis: predictions for θPMNS23 and δPMNS

We revisit mixing sum rule relations in the lepton and quark sectors under the assumption that the 1-3 elements of the flavour mixing matrices (VuL, VdL, VeL, VνL) are zero in the flavour basis. We consider the exact relations resulting from the validity of this “zero 1-3 flavour mixing hypothesis” and analyse their implications based on the current experimental data, including effects from RG running. In particular, we analyse how the existing precise measurement of θPMNS13 allows to derive predictions for θPMNS23 in models with constrained θe12. As examples, we calculate the predictions for θPMNS23 which arise in 12 classes of Pati-Salam models and SU(5) GUTs that relate θe12 to θd12. We also derive a novel “lepton phase sum rule”, valid under the additional assumption of small charged lepton mixing contributions. We furthermore point out that, in the context of GUT flavour models, the quark and lepton CP violating phases δCKM and δPMNS can both be predicted from a single imaginary element in the mass matrices.