Ratio of electric quadrupole to magnetic dipole amplitudes in the nucleon-delta transition

We have analyzed available data on magnetic dipole and electric quadrupole amplitudes for the photoproduction of pions around the delta region, in the framework of a phenomenological {γ}N{Δ} interaction and background. The unitarity constraint via Watson's theorem limits severely the range of allowed gauge couplings in the {γ}N{Δ} vertex. We obtain (-1.5±{}0.2)% as the ratio of the electric quadrupole to magnetic dipole resonant amplitudes, a value of the same order of magnitude as predicted in the skyrmion model of baryons. This ratio suggests a deformed structure of the nucleon and the delta isobar.