The structure of the R8 term in type IIB string theory
Abstract
Based on the structure of the on-shell linearized superspace of type IIB supergravity, we argue that there is a non--BPS 16 derivative interaction in the effective action of type IIB string theory of the form (t8 t8 R4)2, which we call the R8 interaction. It lies in the same supermultiplet as the G8 R4 interaction. Using the KLT relation, we analyse the structure of the tree level eight graviton scattering amplitude in the type IIB theory, which leads to the R8 interaction at the linearized level. This involves an analysis of color ordered multi-gluon disc amplitudes in the type I theory, which shows an intricate pole structure and transcendentality consistent with various other interactions. Considerations of S-duality show that the R8 interaction receives non-analytic contributions in the string coupling at one and two loops. Apart from receiving perturbative contributions, we show that the R8 interaction receives a non-vanishing contribution in the one D-instanton-anti-instanton background at leading order in the weak coupling expansion.