A new effective-one-body Hamiltonian with next-to-leading order spin-spin coupling

TL;DR

This paper introduces a new effective-one-body Hamiltonian for black hole binaries with arbitrarily oriented spins, incorporating next-to-leading order spin-spin coupling more efficiently and simply than previous models.

Contribution

The authors develop a simplified EOB Hamiltonian with fewer terms and a clearer momentum structure, improving modeling of spin-spin interactions in black hole binaries.

Findings

Reduced the NLO spin-spin Hamiltonian to 9 terms from 25.

Achieved a simpler quadratic momentum structure in the Hamiltonian.

Established a gauge fixing that concentrates spin-spin effects into a single potential.

Abstract

We present a new effective-one-body (EOB) Hamiltonian with next-to-leading order (NLO) spin-spin coupling for black hole binaries endowed with arbitrarily oriented spins. The Hamiltonian is based on the model for parallel spins and equatorial orbits developed in [Physical Review D 90, 044018 (2014)], but differs from it in several ways. In particular, the NLO spin-spin coupling is not incorporated by a redefinition of the centrifugal radius $r_c$, but by separately modifying certain sectors of the Hamiltonian, which are identified according to their dependence on the momentum vector. The gauge-fixing procedure we follow allows us to reduce the 25 different terms of the NLO spin-spin Hamiltonian in Arnowitt-Deser-Misner coordinates to only 9 EOB terms. This is an improvement with respect to the EOB model recently proposed in [Physical Review D 91, 064011 (2015)], where 12 EOB terms were involved. Another important advantage is the remarkably simple momentum structure of the spin-spin terms in the effective Hamiltonian, which is simply quadratic up to an overall square root. Moreover, a Damour-Jaranowski-Sch\"afer-type gauge could be established, thus allowing one to concentrate, in the case of circular and equatorial orbits, the whole spin-spin interaction in a single radial potential.