Exchange currents in nucleon electroexcitation -: art. no. 035203

We calculate the scalar and transverse helicity amplitudes for the electromagnetic excitation of nucleon resonances as a function of the photon four-momentum transfer. The helicity amplitudes are decomposed into electromagnetic multipoles and connected to the gammaN-->N* transition form factors. The internal N and N* dynamics is described by a constituent quark model (CQM) Hamiltonian with gluon, pion, and sigma -meson exchange potentials as residual interactions. The N and N*-resonance wave functions are obtained by solving the Schrodinger equation in a harmonic oscillator basis which contains up to N = 2 excitation quanta. For the electromagnetic current we include. in addition to the one-body current, two-body exchange currents associated with the quark-quark potentials. Exchange currents provide an effective description of the cloud of q (q) over bar pairs, which together with the valence quarks are important degrees of freedom in physical hadrons. We obtain sizable contributions of the two-body exchange currents for nearly all gamma NN* amplitudes. For some observables, e.g., the C2/M1 ratio in the gammaN-->Delta (1232) transition, and the M1 transition to the N*( 1440), exchange currents provide the most important contribution.