D-instantons and String Field Theory

Abstract

In this thesis, we study D-instanton contributions to supergraviton scattering amplitudes in ten-dimensional type IIB superstring theory beyond the leading non-perturbative order. Our computation is based on the Neveu-Schwarz-Ramond (NSR) formalism with picture changing operators and vertical integration. In the first chapter, we determine the single D-instanton contribution to maximal R-symmetry violating (MRV) amplitudes with arbitrary momenta at the first subleading order in string coupling, as well as the effects of a D-/anti-D-instanton pair at leading nontrivial order in the momentum expansion. These results confirm a number of predictions of S-duality, and unveil some previously unknown pieces of type IIB string amplitudes. The naive on-shell prescription for D-instanton mediated amplitudes, based on integration over the moduli space of worldsheet geometries as well as that of D-instanton boundary conditions, suffers from potential open string divergences and regularization ambiguities. In the second chapter, we employ the framework of open+closed superstring field theory (SFT) to address such issues. From this, we are able to unambiguously compute a part of the momentum-independent constant that appears at first subleading order in the string coupling, which serves as a highly nontrivial consistency check on spacetime supersymmetry and soft relations.