Manifest colour-kinematics duality and double copy in the string-inspired formalism

Authors: Naser Ahmadiniaz, Filippo Maria Balli, Olindo Corradini, Cristhiam Lopez-Arcos, Alexander Quintero Vélez, Christian Schubert Preprint number: UUITP-47/21 Abstract: The relation for the gravity polarisation as the tensor product of two gluon polarisation vectors has been well-known for a long time, but a version of this relation for multi-particle fields is still not present. Here we show that in order for this to happen we first have to ensure that the multi-particle polarisations satisfy colour-kinematics duality, which arises naturally from the Bern-Kosower formalism for one-loop gluon amplitudes, and later we will see that the tensor product for multi-particle fields arises naturally in the Bern- Dunbar-Shimada formalism for one-loop gravity amplitudes. This allows us to formulate a new prescription for double-copy gravity Berends-Giele currents that can be applied to other cases. To make this point we also present other examples.