# Pion photoproduction off nucleons in covariant chiral perturbation theory

## TL;DR

This paper investigates pion photoproduction near threshold using covariant baryon chiral perturbation theory at O(p^3), explicitly including the Δ(1232) resonance, and compares theoretical predictions with experimental data to constrain low energy constants.

## Contribution

It introduces a covariant chiral perturbation theory approach with explicit Δ(1232) inclusion for pion photoproduction, providing improved data agreement and constraining low energy constants.

## Key findings

- Δ(1232) significantly improves data agreement
- Most low energy constants are well known from other processes
- The theory accurately describes cross sections and polarization observables

## Abstract

Pion photoproduction off the nucleon close to threshold is studied in covariant baryon chiral perturbation theory at O($p^3$) in the extended-on-mass-shell scheme, with the explicit inclusion of the $\Delta(1232)$ resonance using the $\delta$ counting. The theory is compared to the available data of cross sections and polarization observables for all the charge channels. Most of the necessary low energy constants are well known from the analysis of other processes and the comparison with data strongly constrains some of the still unknown ones. The $\Delta(1232)$ contribution is significant in improving the agreement with data, even at the low energies considered.