“…Effective non-gravitational theories for which a double-copy construction is known include the Dirac-Born-Infeld and the special Galileon theories, as well as the nonlinear sigma model [38,39,40,41,42,43,44]. Double-copy structures have also been identified for various string-theory amplitudes, [45,46,47,48,49,50,51,52,53], within the Cachazo-He-Yuan formalism [54,55,56,57,58], in the context of ambitwistor string theories [59], and at the level of linearized supermultiplets [60,61,62,63,64].…”