Pion Cloud and the Q² Dependence of γ^*N ↔ Δ Transition Form Factors

Recent experiments indicate that the behavior exhibited by the ratios E_1⁺/M_1⁺ and S_1⁺/M_1⁺ of the γ^*N↔Δ transition remains small and negative for Q²≤4.0 GeV². 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 γ^*NΔ form factors. Within our model we find that the bare Δ is almost spherical and the electric E2 and Coulomb C2 quadrupole excitations of the physical Δ are nearly saturated by the pion cloud contribution in Q²≤4.0 GeV².