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Abstract

Previous work has shown that massless tree amplitudes of the type I and IIA/B
superstrings can be dramatically simplified by expressing them as double copies
between field-theory amplitudes and scalar disk/sphere integrals, the latter
containing all the $\alpha'$-corrections. In this work, we pinpoint similar
double-copy constructions for the heterotic and bosonic string theories using
an $\alpha'$-dependent field theory and the same disk/sphere integrals.
Surprisingly, this field theory, built out of dimension-six operators such as
$(D_\mu F^{\mu \nu})^2$, has previously appeared in the double-copy
construction of conformal supergravity. We elaborate on the $\alpha'
\rightarrow \infty$ limit in this picture and derive new amplitude relations
for various gauge-gravity theories from those of the heterotic string.