New Phases of String Theory and 6d RG Fixed Points via Branes at Orbifold Singularities

Abstract

We discuss type II and type I branes at general ADE type orbifold singularities. We show that there are new phases of type I or heterotic string theory in six dimensions, involving extra tensor multiplets, which arise when small instantons sit on orbifold singularities. The theories with extra tensor multiplets are explicitly constructed via orientifolds. The world-volume theories in type IIB or type I five-branes at orbifold singularities lead to the existence of several infinite classes of six dimensional, interacting, renormalization group fixed point theories.