We study the effects of the ∆(1232) resonance as an effective degree of freedom for charged and neutral pion photo-production on nucleons. Different observables have been calculated for these processes by using relativistic chiral perturbation theory up to O(p 3 ) in the δ counting, thus, including pion loops. We compare our model with a large database containing the available experimental data and constrain some unknown low energy constants.

Pion electro-and 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 ∆(1232) resonance. The relevant low energy constants are fixed by fitting the available experimental data with the theoretical model. The inclusion of the ∆ resonance as an explicit degree of freedom substantially improves the agreement with data and the convergence of the model.

We discuss some applications of the unquenched quark model which is an extension of the CQM that includes the effects of sea quarks via a 3 P 0 quark-antiquark pair-creation mechanism. Particular attention is paid to the electromagnetic couplings and β decays of baryons. It is shown that the observed discrepancies between the experimental data and the predictions of the CQM can be accounted for in large part by the effects of sea quarks in the unquenched quark model.

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