Phenomenology of String-Inspired Supersymmetric Unified Theories

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Superstring theory has attracted much attention as a candidate for unified theory in higher dimensions. Recently there have been many attempts to explain various problems of supersymmetric unified theories in the framework of higher dimensions. Yanagida and his collaborators have shown that the semi-unification model which can solve the triplet-doublet splitting problem of grand unification can naturally be embedded into higher dimensional type IIB supergravity compactified on an orbifold, and some characteristic features of the model are accounted for in this framework. They have also studied six-dimensional models with grand unified group broken via orbifolding. Furthermore speculations have been given that three family of quarks and leptons may have higher dimensional origin. Besides these studies, they also investigated, e.g. models for neutrino oscillation, leptogenesis in the Universe, and a model to explain dark energy of the Universe. Yamaguchi and his collaborators have proposed a novel mechanism to explain the triplet-doublet splitting and the suppression of proton decay in the framework of extra dimensions. They have also investigated TeV scale extra dimensions accessible to standard model particles. Present experimental contraints are given and possible signatures at LHC experiment are discussed. Furthermore they studied mechanisms of mediation of supersymmetry breakdown and phenomenological consequences.