Kerr binary dynamics from minimal coupling and double copy

TL;DR

This paper develops a new gauge theory framework incorporating classical spin effects, enabling the calculation of Kerr binary dynamics and scattering amplitudes to all orders in spin using double-copy techniques.

Contribution

It introduces a novel Yang-Mills Lagrangian with a classical spin covariant derivative, allowing for spin effects in amplitude computations and gravitational observables.

Findings

Derived a unique contact term structure for the Lagrangian.

Computed the 2PM scattering amplitude for spinning objects to all orders in spin.

Extracted the 2PM scattering angle for Kerr binary systems.

Abstract

We construct a new Yang-Mills Lagrangian based on a notion of minimal coupling that incorporates classical spin effects. The construction relies on the introduction of a new covariant derivative, which we name "classical spin covariant derivative", that is compatible with the three-point interaction of the $\sqrt{\mathrm{Kerr}}$ solution with the gauge field. The resulting Lagrangian, beside the correct three-point coupling, predicts a unique choice for contact terms and therefore it can be used to compute higher-point amplitudes such as the Compton, unaffected by spurious poles. Using double-copy techniques we use this theory to extract gravity amplitudes and observables that are relevant to describe Kerr binary dynamics to all orders in the spin. In particular, we compute the 2PM ($\mathcal{O}(G^2_N)$) $2\rightarrow 2$ scattering amplitude between two classically spinning objects to all orders in the spin and use it to extract the 2PM scattering angle.