We discuss the results of the covariant spectator quark model for the several $$\gamma ^*N \rightarrow N^*$$ γ ∗ N → N ∗ transitions, where N is the nucleon and $$N^*$$ N ∗ a nucleon excitation. More specifically, we present predictions for the form factors and transition amplitudes associated with the resonances $$N(1440)1/2^+$$ N ( 1440 ) 1 / 2 + , $$N(1535)1/2^-$$ N ( 1535 ) 1 / 2 - , $$N(1520)3/2^-$$ N ( 1520 ) 3 / 2 - , $$\varDelta (1620)1/2^-$$ Δ ( 1620 ) 1 / 2 - and $$N(1650)1/2^-$$ 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 ( $$Q^2$$ Q 2 ). In general the estimates are in good agreement with the empirical data for $$Q^2 >2$$ Q 2 > 2 GeV $$^2$$ 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 $$N^*$$ N ∗ . We also review our results for the resonance $$\varDelta (1232)3/2^+$$ Δ ( 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.
Few-Body Systems – Springer Journals
Published: Jun 4, 2018
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