Binary Dynamics Through the Fifth Power of Spin at $\mathcal{O}(G^2)$

TL;DR

This paper advances the understanding of binary systems with arbitrary spins by computing higher-order spin contributions to the two-body Hamiltonian at second order in gravitational constant G, using a scattering amplitudes framework.

Contribution

It introduces a method to evaluate higher spin effects at $ ext{O}(G^2)$, including $S^3$, $S^4$, and $S^5$ contributions, confirming results with aligned-spin cases and proposing a conjecture for Kerr black holes.

Findings

Computed $S^3$, $S^4$, and $S^5$ contributions to the Hamiltonian at $ ext{O}(G^2)$.

Confirmed higher-order spin results against aligned-spin cases.

Proposed a shift symmetry and high-energy growth constraint to determine Kerr black hole coefficients.

Abstract

We use a previously developed scattering-amplitudes-based framework for determining two-body Hamiltonians for generic binary systems with arbitrary spin $S$. By construction this formalism bypasses difficulties with unphysical singularities or higher-time derivatives. This framework has been previously used to obtain the exact velocity dependence of the $\mathcal O(G^2)$ quadratic-in-spin two-body Hamiltonian. We first evaluate the $S^3$ scattering angle and two-body Hamiltonian at this order in $G$, including not only all operators corresponding to the usual worldline operators, but also an additional set due to an interesting subtlety. We then evaluate $S^4$ and $S^5$ contributions at $\mathcal O(G^2)$ which we confirm by comparing against aligned-spin results. We conjecture that a certain shift symmetry together with a constraint on the high-energy growth of the scattering amplitude specify the Wilson coefficients for the Kerr black hole to all orders in the spin and confirm that they reproduce the previously-obtained results through $S^4$.