D-instantons and Effective Couplings in Intersecting D-brane Models

This thesis is concerned with two aspects of intersecting D6-brane
models: We treat some of the effects that are induced by
E2-instantons and we consider one-loop corrections to the gauge
kinetic functions in the low-energy effective field theory. We
begin in chapter 1 with a general introduction covering the
question why to consider string theory as a candidate theory of
nature. After that, in chapter 2, we go over some topics related to
field theory instantons and finally discuss the instanton generated
superpotential in supersymmetric QCD (SQCD), the so-called
Affleck–Dine–Seiberg (ADS) superpotential. In chapter 3 we then
proceed to reproduce the ADS superpotential in an intersecting
D6-brane realization of SQCD as the effect of a single
E2-instanton. In order to do so, we first explicate the necessary
formalism for calculating the E2-instanton contribution to the
superpotential, then construct a (local) intersecting D6-brane
realization of SQCD, and finally show that the formalism indeed
leads to the Affleck–Dine–Seiberg superpotential. In chapter 4 we
consider one-loop corrections to the gauge kinetic functions. These
one-loop corrections are related quite intricately to gauge
threshold corrections which we compute for intersecting D6-brane
models on the Z_2xZ_2 toroidal orbifold. An analysis of the
formulas for calculating the E2-instanton contribution to the
superpotential raises the question whether this contribution is a
holomorphic quantity. In chapter 5 we take up this question showing
that indeed it is holomorphic. In addition, we discuss under which
circumstances one can expect E2-instanton corrections to the gauge
kinetic functions. There follow a number of appendices, some
elaborating on points touched upon in the main text, some
collecting necessary formulas.