On the Lagrangian formulation of the double copy to cubic order

TL;DR

This paper explores the Lagrangian formulation of the double-copy correspondence between gauge theories and gravity up to cubic order, revealing new structures and symmetries that connect these theories and their interactions.

Contribution

It provides a detailed Lagrangian framework for the double-copy construction at cubic order, including gauge fixings and the derivation of cubic vertices for gravity multiplets.

Findings

Free gravitational Lagrangians as products of Yang-Mills Lagrangians

Existence of gauge fixings mixing spin-one sectors

Cubic interactions matching $ abla=0$ supergravity

Abstract

We investigate the Lagrangian formulation of the double-copy correspondence between gauge theories and gravity, up to the cubic order. Building on the definition of the double-copy field as a convolution of two vectors, we obtain free gravitational Lagrangians as products of two Yang-Mills Lagrangians, in a form amenable to be easily extended to the massive case. We discuss the origin of these results from tensionless strings and show the existence of gauge fixings that mix the two spin-one sectors and lead to an alternative, especially simple, version of the free Lagrangian. We then construct cubic vertices for the full double-copy multiplet, comprising a graviton, a two-form and a scalar particle, by means of the Noether procedure. Both at the free and at the cubic level the result gets uniquely fixed only upon imposing, on top of gauge invariance, a left-right Lorentz symmetry ruling contraction of indices among double-copy fields. Whereas the outcome nicely matches the cubic interactions of $\mathcal{N}=0$ supergravity, including the gauge-invariant coupling between the scalar particle and the two-form, such a twofold Lorentz symmetry seems to conflict with the perturbative reconstruction of space-time geometry.