Pion photoproduction off nucleons in covariant chiral perturbation theory

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 Δ ( 1232 ) resonance using the δ 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 constrains some of the still unknown ones. The Δ ( 1232 ) contribution is significant in improving the agreement with data, even at the low energies considered.