String interactions in gravitational wave backgrounds Original Research Article

The non-compact CFT of a class of NS-supported pp-wave backgrounds is solved exactly. The associated tree-level covariant string scattering amplitudes are calculated. The S-matrix elements are well-defined, dual but not analytic as a function of p+. They have poles corresponding to physical intermediate states with p+≠0 and logarithmic branch cuts due to on-shell exchange of spectral-flow images of p+=0 states. When μ→0 a smooth flat space limit is obtained. The μ→∞ limit, unlike the case of RR-supported pp-waves, gives again a flat space theory.