Pion photoproduction off the proton in a gauge-invariant chiral unitary framework

TL;DR

This paper develops a gauge-invariant chiral unitary approach to pion photoproduction off the proton, fitting scattering data to determine low-energy constants and predicting multipole amplitudes without additional parameters.

Contribution

It introduces a novel gauge-invariant chiral framework for pion photoproduction, incorporating next-to-leading order interactions and providing parameter-free predictions for multipole amplitudes.

Findings

Successfully fits pion-nucleon scattering data

Predicts s-wave multipole amplitudes without extra parameters

Quantifies theoretical uncertainties with different fit strategies

Abstract

We investigate pion photoproduction off the proton in a manifestly gauge-invariant chiral unitary extension of chiral perturbation theory. In a first step, we consider meson-baryon scattering taking into account all next-to-leading order contact interactions. The resulting low-energy constants are determined by a fit to s-wave pion-nucleon scattering and the low-energy data for the reaction pi- p --> eta n. To assess the theoretical uncertainty, we perform two different fit strategies. Having determined the low-energy constants, we then analyse the data on the s-wave multipole amplitudes E0+ of pion and eta photoproduction. These are parameter-free predictions, as the two new low-energy constants are determined by the neutron and proton magnetic moments.