The covariant superstring action of Green and Schwarz provides a possible alternative to the more widely used Neveu-Schwarz-Ramond action. It is shown that standard gauge-fixing techniques can be applied to the Green-Schwarz action for the heterotic string, and that the resulting path integral has no conformal anomaly in the critical dimension. The path integral over surfaces with the topology of a cylinder is investigated in some detail, and is demonstrated to give a good tree level propagator. The propagator is Lorentz covariant in spite of the noncovariance of the gauge-fixing technique. © 1987.
Carlip, S. (1987). Heterotic string path integrals with the Green-Schwarz covariant action. Nuclear Physics, Section B, 284(C), 365–385. https://doi.org/10.1016/0550-3213(87)90040-X