θ13PMNS=θC/2 from GUTs

The recent observations of the leptonic mixing angle θ13PMNS are consistent with θ13PMNS=θC/2 (with θC being the Cabibbo angle θ12CKM). We discuss how this relation can emerge in Grand Unified Theories (GUTs) via charged lepton corrections. The key ingredient is that in GUTs the down-type quark Yukawa matrix and the charged lepton Yukawa matrix are generated from the same set of GUT operators, which implies that the resulting entries are linked and differ only by group-theoretical Clebsch factors. This allows a link θ12e≈θC to be established, which can induce θ13PMNS≈θC/2 provided that the 1-3 mixing in the neutrino mass matrix is much smaller than θC. We find simple conditions under which θ13PMNS≈θC/2 can arise via this link in SU(5) GUTs and Pati-Salam models. We also discuss possible corrections to the exact relation. Using lepton mixing sum rules different neutrino mixing patterns can be distinguished by their predictions for the Dirac CP phase δPMNS.