Color-kinematics dual representations of one-loop matrix elements in the open-superstring effective action

TL;DR

This paper explores the construction of color-kinematics dual numerators at one loop in open superstring theory, revealing limitations and obstructions related to certain operator insertions and their implications for the double copy construction.

Contribution

It constructs one-loop color-kinematics dual numerators for specific string operator insertions and identifies obstructions and incompatibilities in achieving full duality.

Findings

Successfully constructed numerators for $oldsymbol{ ext{zeta}_3}$, $oldsymbol{ ext{zeta}_5}$, and $oldsymbol{ ext{zeta}_3^2}$ insertions.

Identified an obstruction in representing $oldsymbol{ ext{zeta}_7}$ insertions with full color-kinematics duality.

Demonstrated double copy constructions that match cut kinematics despite duality obstructions.

Abstract

The $\alpha'$-expansion of string theory provides a rich set of higher-dimension operators, indexed by $\zeta$ values, which can be used to study color-kinematics duality and the double copy. These two powerful properties, actually first noticed in tree-level string amplitudes, simplify the construction of both gauge and gravity amplitudes. However, their applicability and limitations are not fully understood. We attempt to construct a set of color-kinematics dual numerators at one loop and four points for insertions of operator combinations corresponding to the lowest four $\zeta_2$-free operator insertions from the open superstring: $\zeta_3$, $\zeta_5$, $\zeta_3^2$, and $\zeta_7$. We succeed in finding a representation for the first three in terms of box, triangle, and bubble numerators. In the case of $\zeta_7$ we find an obstruction to a fully color-dual representation related to the bubble-on-external-leg type diagrams. We discuss two paths around this obstruction, both of which signal an incompatability between color-kinematics duality and manifesting certain desired properties. Using the constructed color-dual numerators, we find two different Bern-Carrasco-Johansson double copies that produce candidate closed-string-insertion numerators. Both approaches to the double copy match the kinematics of the cuts, with relative normalization set by either summing over both double copies including degeneracy or by including an explicit prefactor on the double-copy numerator definitions.