Unified theories of elementary particles based on supersymmetric string models

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We have investigated various problems in constructing unified theories of elementary particles based on supersymmetric string models.Supersymmetry was proposed to solve the gauge hierarchy problem in grand unified theories. In order to include gravity into the unified theory, superstring theories are most promising. We have examined the so-called Hawking radiation in string theories. More concretely, we put strings in a strong uniform gravitational field and observed that there is an upper limit above which the gravitational field cannot be made stronger because of violent particle production in the strong gravitational field.At low energies, superstring theories look like a system of supersymmetric Yang-Mills fields coupled to supergravity fields with systematic corrections in powers of the slope parameter and the string loop expansion parameter. We have explicitly worked out the one-loop correction (torus topology) to the effective Lagrangian in the case of heterotic string as well as type II superstring.