One-loop amplitudes and effective action in superstring theories

Abstract

Four graviton and/or gauge boson amplitudes in the heterotic string and type II superstring theories are explicitly computed in the operator formalism to the one-loop order (corresponding to the torus topology in the Polyakov approach). It is shown that the symmetry factor 1 2 is needed in order to satisfy the unitarity constraint. The effective action to order (α′) 3 is found to receive the string loop corrections whose parameters are fixed already at the string tree level.