Q2-evolution of nucleon-to-resonance transition form factors in a QCD-inspired vector-meson-dominance model

Abstract

We adopt the vector-meson-dominance approach to investigate ${Q}^{2}$-evolution of $NR$-transition form factors ($N$ denotes nucleon and $R$ an excited resonance) in the first and second resonance regions. The developed model is based upon conventional $\ensuremath{\gamma}NR$-interaction Lagrangians, introducing three form factors for spin-$3/2$ resonances and two form factors for spin-$1/2$ nucleon excitations. Lagrangian form factors are expressed as dispersionlike expansions with four or five poles corresponding to the lowest excitations of the mesons $\ensuremath{\rho}(770)$ and $\ensuremath{\omega}(782)$. Correct high-${Q}^{2}$ form-factor behavior predicted by perturbative QCD is due to phenomenological logarithmic renormalization of electromagnetic coupling constants and linear superconvergence relations between the parameters of the meson spectrum. The model is found to be in good agreement with all the experimental data on ${Q}^{2}$-dependence of the transitions $N\ensuremath{\Delta}(1232)$, $NN(1440)$, $NN(1520)$, $NN(1535)$. We present fit results and model predictions for high-energy experiments proposed by JLab. Besides, we make special emphasis on the transition to perturbative domain of $N\ensuremath{\Delta}(1232)$ form factors.