Nonperturbative formulation of string theory by fermions and the study of black holes

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The purpose of this research is to construct a nonperturbative formulation of string theory by studying it from the viewpoints of solitons. D-branes. fermions. singularities of spacetime and black holes.First, we suggest that the boundary cosmological constant ζ in c<1 unitary string theory be regarded as the one-dimensional complex coordinate of the target space on which the boundaries of world-sheets can live. From this viewpoint we explicitly construct analogues of D-instantons which satisfy Polchinsk's "combinatories of boundaries." We further show that our operator formalism developed in the preceding articles is powerful in evaluating D-instanton effects, and also demonstrate for simple cases that these effects exactly coincide with the stringy nonperturbative effects found in the exact solutions of string equations.Next, we investigate how much a first-quantized charged bosonic test string gets excited after crossing a shock wave generated by a charged particle with mass M^^- and charge Q^^-. The shock wave is given by a charged Aichelburg-Sexl (CAS) spacetime where Q^^- = 0 corresponds to the ordinary Aichelburg-Sexl one. We compute the mass expectation value of the charged test string after passing through the shock wave in the CAS spacetime. In the case of small Q^^-, gravitational and Coulomb forces are canceled out each other and hence the excitation of the string remains very small. In the case of large Q^^-, however, every charged string gets highly excited by quantum fluctuation in the extra-dimension caused by both the gauge and the axion fields.We have been also studying the enhancon mechanism for the D6-D2 system.