Lucile Cangemi: From Quantum to Classical Scattering of Kerr Black Holes: A construction of massive higher-spin scattering amplitudes and their classical limits.
- Date: 21 May 2024, 09:00
- Location: Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala
- Type: Thesis defence
- Thesis author: Lucile Cangemi
- External reviewer: Yu-tin Huang
- Supervisor: Henrik Johansson
- Research subject: Theoretical Physics
- DiVA
Abstract
Gravitational scattering processes involving black holes as asymptotic states can provide insight into the classical dynamics of binary black hole systems. The observed gravitational waves emitted during mergers need to be compared to high-precision theoretical predictions. By modelling black holes as massive point particles in an effective quantum field theory, one can take advantage of the advanced computational tools originally designed for collider physics. For Schwarzschild black holes the natural objects to study are scattering amplitudes involving massive scalar fields with interactions mediated by gravitons. The classical physics is extracted by considering limits of the kinematics.
Extending this effective description to rotating Kerr black holes introduces subtleties. To leading order in the post-Minkowskian perturbation scheme, there now exists candidate three-point scattering amplitudes for massive higher-spin particles that in the classical limit reproduce the Kerr metric. For small quantum spins, these are given by familiar theories of interacting massive fields which have a well-behaved massless limit. These theories are sufficient to capture the first few spin-multipole orders for the classical observables; however, to capture more orders one is required to use input from higher-spin theories. The three-point higher-spin amplitudes were originally introduced without reference to an underlying Lagrangian description. Lagrangians for interacting higher-spin fields are notoriously complicated as they necessarily describe composite fields in an effective higher-derivative theory.
This thesis explores the underlying higher-spin effective theories suitable for describing rotating black holes, and proposes a new spin-s family of Compton scattering amplitudes. We present two complementary constructions for consistent interacting higher-spin Lagrangians: the first relies on massive higher-spin gauge symmetry to remove unwanted states, and the second one manifests the correct degrees of freedom using a chiral field framework. A significant portion of the thesis discusses how to extract classical physics from quantum amplitudes, focusing on consistent treatments of the spin degrees of freedom. The resulting quantum and classical Compton amplitudes are built to be consistent with perturbations of the Kerr metric, through a combination of constraints from higher-spin considerations and classical analysis.
In addition to the black-hole amplitudes, we study the scattering of higher-spin fields in a gauge theory referred to as root-Kerr. The three point amplitudes of this gauge theory are closely related to the Kerr ones, such that it provides an instructive model for both higher-spin consistency and classical analysis. Another toy model discussed is the scattering of higher-spin superstring states on the leading Regge trajectory.