Neutrino mixing and quark-lepton complementarity

As a result of the identification of the solution to the solar neutrino problem, a rather precise relation ${\ensuremath{\theta}}_{\mathrm{s}\mathrm{u}\mathrm{n}}+{\ensuremath{\theta}}_{C}=\ensuremath{\pi}/4$ between the leptonic 1-2 mixing angle ${\ensuremath{\theta}}_{\mathrm{s}\mathrm{u}\mathrm{n}}$ and the Cabibbo angle has emerged. It would mean that the lepton and the quark mixing angles add up to the maximal, suggesting a deep structure by which quarks and leptons are interrelated. We refer to the relation as ``quark-lepton complementarity'' (QLC) in this paper. We formulate general conditions under which the QLC relation is realized. We then present several scenarios which lead to the relation and elaborate on phenomenological consequences which can be tested by the future experiments. We also discuss implications of the QLC relation for the quark-lepton symmetry and the mechanism of neutrino mass generation.