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 gamma*N <-> Delta transition remains small and negative for Q(2) less than or equal to 4.0 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 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 the pion cloud contribution in Q(2) less than or equal to 4.0 GeV(2).