Searching for Kerr in the 2PM amplitude

Understand

The classical scattering of spinning objects is well described by the spinor-helicity formalism for heavy particles.

• Using these variables, we derive spurious-pole-free, all-spin opposite-helicity Compton amplitudes (factorizing on physical poles to the minimal, all-spin three-point amplitudes of ref.
• \cite{Arkani-Hamed:2017jhn}) in the classical limit for QED, QCD, and gravity.
• The cured amplitudes are subject to deformations by contact terms, the vast majority of whose contributions we can fix by imposing a relation between spin structures -- motivated by lower spin multipoles of black hole scattering -- at the second post-Minkowskian (2PM) order.