Spin observables in pion photoproduction from a unitary and causal effective field theory

TL;DR

This paper analyzes pion photoproduction using a chiral Lagrangian, employing a method that respects gauge invariance, causality, and unitarity to accurately extrapolate reaction amplitudes and predict spin observables up to 1300 MeV.

Contribution

It introduces a novel approach combining chiral perturbation theory with analytic extrapolation constrained by fundamental principles to describe pion photoproduction data.

Findings

Reproduces experimental data up to 1300 MeV

Provides predictions for various spin observables

Ensures gauge invariance, causality, and unitarity in the analysis

Abstract

Pion photoproduction is analyzed with the chiral Lagrangian. Partial-wave amplitudes are obtained by an analytic extrapolation of subthreshold reaction amplitudes computed in chiral perturbation theory, where the constraints set by electromagnetic-gauge invariance, causality and unitarity are used to stabilize the extrapolation. The experimental data set is reproduced up to energies $\sqrt{s}\simeq 1300$ MeV in terms of the parameters relevant at order $Q^3$. We present and discuss predictions for various spin observables.