Pion Cloud and theQ2Dependence ofγ*N↔ΔTransition Form Factors

Recent experiments indicate that the behavior exhibited by the ratios ${E}_{{1}^{+}}/{M}_{{1}^{+}}$ and ${S}_{{1}^{+}}/{M}_{{1}^{+}}$ of the ${\ensuremath{\gamma}}^{*}N\ensuremath{\leftrightarrow}\ensuremath{\Delta}$ transition remains small and negative for ${Q}^{2}\ensuremath{\le}4.0{\mathrm{GeV}}^{2}$. It implies that perturbative QCD is still not applicable at these momentum transfers. We show that these data can be explained in a dynamical model for electromagnetic production of pions, together with a simple scaling assumption for the bare ${\ensuremath{\gamma}}^{*}N\ensuremath{\Delta}$ form factors. Within our model we find that the bare $\ensuremath{\Delta}$ is almost spherical and the electric $E2$ and Coulomb $C2$ quadrupole excitations of the physical $\ensuremath{\Delta}$ are nearly saturated by the pion cloud contribution in ${Q}^{2}\ensuremath{\le}4.0{\mathrm{GeV}}^{2}$.