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 ${\gamma }^{\ast }N\Delta$ 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 $Q^2\le 4.0Ge{V}^2$.

8 pages, 2 figures, to appear in the Proc. of "NSTAR2000 - The Physics of Excited Nucleons", Feb. 16-19, 2000, Jefferson Lab, USA, World Scientific, Singapore, 2000