$N^{\ast }$ 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.

Proceedings of the 11th International Workshop on the Physics of Excited Nucleons (NSTAR 2017), August 20-23, 2017, Columbia, USA. Published version. 29 pages, 15 figures. Typos corrected. Section 4 extended

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• Hyperon electromagnetic timelike elastic form factors at large $q^2$
• Transition form factors and helicity amplitudes for electroexcitation of negative- and positive parity nucleon resonances in a light-front quark model
• Electromagnetic form factors of the ${\Omega }^{-}$ baryon in the spacelike and timelike regions
• Covariant model for the Dalitz decay of the $N(1535)$ resonance
• A covariant model for the decuplet to octet Dalitz decays
• Octet baryon electromagnetic form factor double ratios $(G_E^{\ast }/G_M^{\ast })/(G_E/G_M)$ in a nuclear medium
• Electromagnetic $|G_E/G_M|$ ratios of hyperons at large timelike $q^2$
• Combined parametrization of the neutron electric form factor and the ${\gamma }^{\ast }N\to \Delta (1232)$ quadrupole form factors
• Weak interaction axial form factors of the octet baryons in nuclear medium
• Meson cloud contributions to the Dalitz decays of decuplet to octet baryons
• Electroweak form factors of baryons in dense nuclear matter
• Quark model calculations of transition form factors at high photon virtualities
• Electric and magnetic timelike form factors of hyperons at large transfer momentum
• Octet baryon electroweak form factors in dense nuclear matter