Enhanced Gauge Symmetry in Three-Modulus Models of Type II String and Hypergeometric Series

Abstract

The conifold singularities in the type II string are considered as the points of phase transition. In some cases, these singularities can be understood, in the framework of the conventional field theories, as the points of enhanced gauge symmetry. We consider a class of three-modulus type II strings. It is shown the periods can be written in the form of hypergeometric series around the singular points in these models. The leading expansion around the conifold locus turns out to be described by Appell functions. In one singular point, we observe the enhanced gauge symmetry of SU(2) × SU(2) independent of the models. Around another conifold locus, however, the resulting expression of the Appell functions depends on the models. We examine the result by considering a relation between these Appell functions and underlying Riemann surfaces.