Worldsheet approach to heterotic instantons and solitons

Instantons and soliton solutions of heterotic string theory are investigated with an emphasis on the worldsheet point of view. The solitons have the structure of a fivebrane in ten dimensions, the instantons are simply related to the solitons by Wick rotation. Some new fivebrane solutions are presented with a four-dimensional cross-section of the fivebrane given by an infinitely long, semi-wormhole at the core stabilized by axion charge and containing non-abelian gauge excitations. The generic such solution is shown to correspond to a sigma model with (4, 0) worldsheet supersymmetry. At special symmetric points in the moduli space of multi-instanton or multi-soliton solutions, the spin connection with torsion becomes identical to the Yang-Mills gauge connection. This results in (4, 4) worldsheet supersymmetry, and we argue that the resulting sigma models with torsion are finite and conformally invariant with no α′ corrections. It is further shown that the wormhole throat is then described by an exactly soluble conformal field theory which is essentially a Wess-Zumino-Witten model whose level is related to the axion charge. Because of the simplicity and exactness of these symmetric solutions, they should provide a useful starting point for the analysis of such issues as integration of collective coordinates in string theory, duality between strings and fivebranes and the quantization of fundamental fivebranes.