Quark-lepton complementarity in unified theories

As pointed out by many authors, recent observations are consistent with an intriguing relation between the Cabibbo angle theta(C) and the solar neutrino mixing angle theta(12), namely theta(12) similar or equal to pi/4 - theta(C). Such quark-lepton complementarity (QLC) may be a signal of an underlying quark-lepton unification at short distances. We discuss possible ways to realize this relation in realistic quark-lepton unification theories by identifying a minimal set of operators that lead to QLC while remaining consistent with other known data. The purpose of this Letter is to present the first elements of a unified model at the GUT scale capable of predicting the QLC relation. A generic prediction of our proposed class of models is the new relation for the lepton mixing angle theta(13) similar or equal to theta(C), which allows these models to be confirmed or excluded by the current generation of neutrino oscillation experiments. (c) 2005 Published by Elsevier B.V.