The simplest massive S-matrix: from minimal coupling to Black Holes

TL;DR

The paper develops an on-shell framework for electromagnetically and gravitationally coupled massive higher-spin states, identifying the simplest three-point amplitude as minimal coupling whose UV completion matches minimal derivatives and whose IR behavior yields the classical moment $g=2$ for all spins. By constructing gravitational Compton amplitudes through consistent factorization and mapping to a one-body EFT, the authors show that deformations beyond minimal coupling are forbidden in gravity, and that gauge-gravity double-copy relations require charged states to have $g=2$ as well. They establish a Kerr black hole correspondence by matching the large-spin limit to the one-body EFT, interpreting this as an on-shell manifestation of the no-hair theorem, and they compute the spin-dependent pieces of the classical gravitational potential at 2PM order up to quartic order in spin using the leading singularity in the holomorphic classical limit. The results provide new, explicit spin-dependent corrections to black hole interactions, clarify the role of polynomial ambiguities in higher-spin Compton amplitudes, and reinforce the deep connection between minimal coupling, Kerr black holes, and long-range gravitational scattering within an on-shell formalism.

Abstract

In this paper, we explore the physics of electromagnetically and gravitationally coupled massive higher spin states from the on-shell point of view. Starting with the three-point amplitude, we focus on the simplest amplitude which is characterized by matching to minimal coupling in the UV. In the IR such amplitude leads to g = 2 for arbitrary charged spin states, and the best high energy behavior for a given spin. We proceed to construct the (gravitational) Compton amplitude for generic spins. We find that the leading deformation away from minimal coupling, in the gravitation sector, will lead to inconsistent factorizations and are thus forbidden. As the corresponding deformation in the gauge sector encodes the anomalous magnetic dipole moment, this leads to the prediction that for systems with gauge2 =gravity relations, such as perturbative string theory, all charged states must have g = 2. It is then natural to ask for generic spin, what is the theory that yields such minimal coupling. By matching to the one body effective action, remarkably we verify that for large spins, the answer is Kerr black holes. This identification is then an on-shell avatar of the no hair theorem. Finally using this identification as well as the newly constructed Compton amplitudes, we proceed to compute the spin dependent pieces for the classical potential at 2PM order up to degree four in spin operator of either black holes.

Figures (2)

• Figure 1: A graphical representation of gluing two 3pt amplitudes.
• Figure 2: The triple-cut diagram for $b$-topology triangle cut. The $a$-topology diagram is obtained by exchanging the labels of particles $a$ and $b$.