Background field approach to electromagnetic properties of baryons

We investigate the self-energies of particles in an external magnetic field B. The dependence is generally of the type root P(B) with P a polynomial in B and the participating masses. The nonanalytic point depends on the masses and charges of the involved particles and is unproblematic for stable particles but constrains the linear energy shift approximation for resonances. We recover an earlier reported condition when the self-energy can be expanded in B and derive two more conditions. Furthermore, we obtain the B dependent self-energies of the nucleon and Delta(1232)-isobar in the SU(2) covariant chiral perturbation theory.