SU(6) breaking effects in the nucleon elastic electromagnetic form factors

2000/06/13 by F. Cardarelli, Fabio Cardarelli, Silvano Simula · 6 citations
Physics and Astronomy · #Condensed matter physics #Constituent quark #High-Energy Particle Collisions Research #Nucleon #Particle physics #Particle physics theoretical and experimental studies #Physics #Quantum Chromodynamics and Particle Interactions #Quantum electrodynamics #Quantum mechanics #Quark #Quark model #Quarkonium #Spins #Symmetry breaking #Wave function #hep-ph #nucl-th

paper · pdf · doi:10.1103/physrevc.62.065201

published as Phys.Rev. C62 (2000) 065201 · latex 27 pp., 7 postscript figures

arxiv created 2000/06/13 · openalex publication_date 2000/11/02 · arxiv updated 2009/11/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05

Abstract

The effects of both kinematical and dynamical SU(6) breaking on the nucleon elastic form factors, G_E^N(Q²) and G_M^N(Q²), are investigated within the constituent quark model formulated on the light front. The investigation is focused on G_Eⁿ(Q²) and the ratio G_M^p(Q²)/G_Mⁿ(Q²), which within the SU(6) symmetry are given by G_Eⁿ(Q²)=0 and G_M^p(Q²)/G_Mⁿ(Q²)=\ensuremath-3/2, respectively. It is shown that the kinematical SU(6) breaking caused by the Melosh rotations of the quark spins as well as the dynamical SU(6) breaking due to the mixed-symmetry component generated in the nucleon wave function by the spin-dependent terms of the quark-quark interaction, can affect both G_Eⁿ(Q²) and G_M^p(Q²)/G_Mⁿ(Q²). The calculated G_Eⁿ(Q²) is found to be qualitatively consistent with existing data, though only \ensuremath≃65% of the experimental neutron charge radius can be explained without invoking effects from possible nonvanishing sizes of the constituent quarks and/or many-body currents. At the same time the predictions for the magnetic ratio G_M^p(Q²)/G_Mⁿ(Q²) turn out to be inconsistent with the data. It is, however, shown that the calculations of G_M^N(Q²) based on different components of the one-body electromagnetic current lead to quite different results. In particular, the calculations based on the plus component are found to be plagued by spurious effects related to the loss of the rotational covariance in the light-front formalism. These unwanted effects can be avoided by considering the transverse y component of the current. In this way our light-front predictions are found to be consistent with the data on both G_Eⁿ(Q²) and G_M^p(Q²)/G_Mⁿ(Q²). Finally, it is shown that a suppression of the ratio G_E^p(Q²)/G_M^p(Q²) with respect to the dipole-fit prediction can be expected in the constituent quark model provided the relativistic effects generated by the Melosh rotations of the constituent spins are taken into account.

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