Superstring Theory and Low-Energy Physics

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Superstring theory is the most promising unified theory. In this theory the space-time dimension should be more than four so that the theory becomes consistent at quantum level. Four-dimensional effective theory is derived after the compactification through which the extra-dimensional space becomes a conformal invariant comactified manifold dynamically. In this theory topological and geometrical structure of the manifold is closely related to various properties of the low-energy effective theory such as gauge symmetries, matter fields and their couplings. Therefore, it is of critical importance to clarify the relation between the structure of compactified manifold and low-energy effective theory. In this proposed spontaneous deformation of conformal invariant manifold and using this mechanism we investigated viable effective theory from calabi-Yau superstring. We studied characteristic features of the effective theory and expected new phennomena at the electroweak scale.1. We found realistic solutions in which the proton stability is guaranteed and the unification of gauge couplings is realized.2. We pointed out the possibility that R-parity is spontaneously broken at TeV scale. Chargino and neutralino mass spectra are calculated and the lightest masses among them are predicted to be less than weak boson masses. Furthemore, we discussed the generation number and the structure of flavor mixing.3. In the two-Higgs-doublet model expected from the supersymmetric model, it is found that the charged Higgs contribution can give a sizable enhancement to the branching fractions of B-meson charamless decay.4. We studied the stability of quantum chiral solitions coupled with the vector mesons.