A Dynamical Model for the Resonant Multipoles and the $\Delta$ Structure

Abstract

We show that recent experiment data for the ratios $E_{1^+}/M_{1^+}$ and $S_{1^+}/M_{1^+}$ can be explained in a dynamical model for electromagnetic production of pions, together with a simple scaling assumption for the bare $\gamma^* N \Delta$ form factors. Within our model we find that the bare $\Delta$ is almost spherical and the electric E2 and Coulomb C2 quadrupole excitations of the physical $\Delta$ are nearly saturated by pion cloud contribution in $Q^2 \le 4.0 GeV^2$.