General Form Factors from String Amplitudes

Abstract

We extend the string-amplitude construction of single-insertion form factors to multiple operator insertions carrying independent off-shell momenta. By deforming the bulk closed-string punctures onto the open-string boundary, we establish a relation between stringy general form factors and scattering amplitudes. Using this relation, we evaluate the field-theory limit and find that the resulting form factors receive contributions from both separate operator insertions and local contact terms. Furthermore, we uncover several hidden structures of stringy general form factors at finite $α'$: $2$-splits and hidden zeros analogous to those of scattering amplitudes, bulk zeros specific to form factors, and a decoupling split unique to multiple insertions.