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 $\gamma \overrightarrow{N}{N}^{*}$ 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 Schrödinger equation in a harmonic oscillator basis which contains up to $N=2\mathrm{}$ 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 $q\overline{q}$ 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 $\gamma {\mathrm{NN}}^{*}$ amplitudes. For some observables, e.g., the $C2/M1\mathrm{}$ ratio in the $\gamma \overrightarrow{N}\Delta \mathrm{}\left(1232\right)\mathrm{}$ transition, and the $M1\mathrm{}$ transition to the $N^{*}\mathrm{}\left(1440\right),$ exchange currents provide the most important contribution.

• Received 19 March 2001

DOI:https://doi.org/10.1103/PhysRevC.64.035203