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Abstract

We investigate one-loop four-point scattering of non-abelian gauge bosons in
heterotic string theory and identify new connections with the corresponding
open-string amplitude. In the low-energy expansion of the heterotic-string
amplitude, the integrals over torus punctures are systematically evaluated in
terms of modular graph forms, certain non-holomorphic modular forms. For a
specific torus integral, the modular graph forms in the low-energy expansion
are related to the elliptic multiple zeta values from the analogous open-string
integrations over cylinder boundaries. The detailed correspondence between
these modular graph forms and elliptic multiple zeta values supports a recent
proposal for an elliptic generalization of the single-valued map at genus zero.