| Summary: | 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 α′-corrections. In this work, we pinpoint similar double-copy constructions for the heterotic and bosonic string theories using an α ′ -dependent field theory and the same disk/sphere integrals. Surprisingly, this field theory, built out of dimension-six operators such as (D μ F μν )2, has previously appeared in the double-copy construction of conformal supergravity. We elaborate on the α ′ → ∞ limit in this picture and derive new amplitude relations for various gauge-gravity theories from those of the heterotic string.
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