Electromagnetic couplings of the chiral perturbation theory Lagrangian from the perturbative chiral quark model

We apply the perturbative chiral quark model to the study of the low-energy πN interaction. Using an effective chiral Lagrangian we reproduce the Weinberg-Tomozawa result for the S-ware πN scattering lengths. After inclusion of the photon field we give predictions for the electromagnetic O(p²) low-energy couplings of the chiral perturbation theory effective Lagrangian that define the electromagnetic mass shifts of nucleons and first-order (e²) radiative corrections to the πN scattering amplitude. Finally, we estimate the leading isospinbreaking correction to the strong energy shift of the π^-p atom in the 1s state, which is relevant for the experiment "pionic hydrogen" at PSI.