$N^\ast$ Form Factors based on a Covariant Quark Model

Abstract

We discuss the results of the covariant spectator quark model for the several $\gamma^\ast N \to N^\ast$ transitions, where $N$ is the nucleon and $N^\ast$ a nucleon excitation. More specifically, we present predictions for the form factors and transition amplitudes associated with the resonances $N(1440)1/2^+$, $N(1535)1/2^-$, $N(1520)3/2^-$, $\Delta(1620)1/2^-$ and $N(1650)1/2^-$. The estimates based on valence quark degrees of freedom are compared with the available data, particularly with the recent Jefferson Lab data at low and large momentum transfer ($Q^2$). In general, the estimates are in good agreement with the empirical data for $Q^2 >2$ GeV$^2$, with a few exceptions. The results are discussed in terms of the role of the valence quarks and the meson cloud excitations for the different resonances $N^\ast$. We also review our results for the resonance $\Delta(1232)3/2^+$ and discuss the relevance of the pion cloud component for the magnetic dipole form factor and the electric and Coulomb quadrupole form factors.