TUNL Lecture Series on Compton Scattering - Chiral EFT, Data and QCD: Where We Are
Compton scattering probes the symmetries and strengths of nucleonic and nuclear interactions and provides a bridge between data and lattice-QCD computations of fundamental hadron properties. The polarizabilities of the nucleon parametrize its stiffness against deformations in electric and magnetic fields. In these lectures, I first review the scattering of light by objects in classical electrodynamics. I introduce Effective Field Theories (EFTs) as the modern tool to describe the Natural World by a separation between short- and long-range phenomena. Nuclear Physics uses Chiral EFT, the extension of Chiral Perturbation Theory to one- and few-nucleon systems. I present formalism, recipes and limitations, and how to quantify theory uncertainties by Bayesian techniques. In the second week, I discuss the theory to extract the polarizabilities of the proton and neutron from data. We confront the results with lattice-QCD computations, and explore the impact on the neutron-proton mass difference and the Anthropic Principle. The spin polarizabilities parametrize the stiffness of the spin (nucleonic bi-refringence/Faraday effect) and probe the spin-dependent component of the pion-nucleon interaction, dictated by chiral symmetry. I review how sensitive observables for each nucleon, the deuteron and 3He are on each scalar and spin polarizability, and to their combinations.