Target space duality in string theory

By exploiting recent arguments about stable nonsupersymmetric D-brane states, we argue that D-brane charge takes values in the K-theory of spacetime, as has been suspected before. In the process, we gain a new understanding of some novel objects proposed recently - such as the Type I zerobrane - and we describe some new objects - such as a -1-brane in Type I superstring theory.

D-Branes And K-Theory

D-brane Approach to Black Hole Quantum Mechanics

Certain two dimensional topological field theories can be interpreted as string theory backgrounds in which the usual decoupling of ghosts and matter does not hold. Like ordinary string models, these can sometimes be given space-time interpretations. For instance, three-dimensional Chern-Simons gauge theory can arise as a string theory. The world-sheet model in this case involves a topological sigma model. Instanton contributions to the sigma model give rise to Wilson line insertions in the space-time Chern-Simons theory. A certain holomorphic analog of Chern-Simons theory can also arise as a string theory.

https://arxiv.org/pdf/hep-th/9207094v2.pdf

Chern-Simons Gauge Theory As A String Theory

Some exact solutions of string theory in four and five dimensions

Equivariant topological sigma models

I present a new class of topological string theories, and discuss them in two dimensions as candidates for the string description of large-N QCD. The starting point is a new class of topological sigma models, whose path integral is localized to the moduli space of harmonic maps from the worldsheet to the target. The Lagrangian is of fourth order in worldsheet derivatives. After gauging worldsheet diffeomorphisms in this “harmonic topological sigma model,” we obtain a topological string theory dominated by minimal-area maps. The bosonic part of this “topological rigid string” Lagrangian coincides with the Lagrangian proposed by Polyakov for the QCD string in higher dimensions.

Petr Hořava

Papers

Some exact solutions of string theory in four and five dimensions

Equivariant topological sigma models

Topological Strings and QCD in Two Dimensions

Two dimensional stringy black holes with one asymptotically flat domain

Two-dimensional string theory and the topological torus