String theory in target space

Understand

It is argued that the complete S-matrix of string theory at tree level in a flat background can be obtained from a small set of target space properties, without recourse to the worldsheet description.

• The main non-standard inputs are (generalised) Britto-Cachazo-Feng-Witten shifts, as well as the monodromy relations for open string theory and the Kawai-Lewellen-Tye relations for closed string theory.
• The roots of the scattering amplitudes and especially their appearance in the residues at the kinematic poles are central to the story.
• These residues determine the amplitudes through on-shell recursion relations.