Superspace Effective Actions for 4D Compactifications of Heterotic and Type II Superstrings

Two-dimensional sigma models are defined for the new manifestly spacetime supersymmetric description of four-dimensional compactified superstrings. The resulting target-superspace effective action is constrained by the way the spacetime dilaton couples to the worldsheet curvature: For the heterotic superstring, the worldsheet curvature couples to the real part of a chiral multiplet, and for Type II it couples to the real part of the sum of a vector multiplet and a tensor hypermultiplet. For the Type II superstring, this contradicts the standard folklore that only a hypermultiplet counts string-loops, explains the peculiar dilaton coupling of Ramond-Ramond fields, and allows the effective action to be easily written in N=2 4D superspace. It also implies that vector multiplet interactions get no quantum corrections, while hypermultiplet interactions can only get corrections if mirror symmetry is non-perturbatively broken.