Higgs Particle Hunting and Attempts going beyond the Standard Model

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The decay rates of Higgs boson into gauge boson pairs were evaluated in the two-Higgs doublet model including radiative corrections. We have found that the effects of heavy unknown particles through radiative corrections are always subleading. This is a generalization of the screening thorem. One-loop corections to the elastic scattering of gauge bosons have also been evaluated in the two-Higgs doublet model.The decay processes of the bottom quark such as b→sl^+l^-, b→svv^^- have been calculated in the minimal supergravity model and multi-Higgs doublet model. It was found in the supergravity model that the branching ratio of the decay b→sl^+l^- could be much larger than in the standard model.To get signals of lepton-flavor violation in accelerator experiments, we have investigated the processes μ^+μ^- (e^+e^-)→τμ+4jets+missing-energy, and μ^+μ^- (e^+e^-)→τμl+2jets+missing energyTo probe the theory beyond the standard model, we have ventured to study various aspects of quantum field theories and superstring. More concretely, we have investigated superconformal invariance realized in particular points in the moduli space of the Seiberg-Witten theory. We have also worked out the derivation of the asymptotic conformal invariance of the three-dimensional anti-de Sitter gravity based on the Chern-Simons theory, and the black hole entropy in the conformal field theory approach. As to the study of superstring, our research includes the investigation of the N=2 QCD from the viewpoints of M-theory, second quantization of the D5-brane, and the Hanany-Witten effects and its relation to the determination of the charge of D2-brane.