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

Abstract

The effects of both kinematical and dynamical SU (6) breaking on the nucleon elastic form factors, G N E (Q 2 ) and G N M (Q 2 ), are investigated within the constituent quark model formulated on the light-front. The investigation is focused on G n E (Q 2 ) and the ratio G p M (Q 2 )/G n M (Q 2 ), which within the SU (6) symmetry are given by G n E (Q 2 ) = 0 andIt 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 bothis found to be qualitatively consistent with existing data, though only ≃ 65% of the experimental neutron charge radius can be explained without invoking effects from possible non-vanishing sizes of the constituent quarks and/or many-body currents. At the same time the predictions for the magnetic ratio G p M (Q 2 )/G n M (Q 2 ) turn out to be inconsistent with the data. It is however shown that the calculations of G N M (Q 2 ) 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 n E (Q 2 ) and GFinally, it is shown that a suppression of the ratio G p E (Q 2 )/G p M (Q 2 ) 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.

Neural network parameterizations of electromagnetic nucleon form-factors

“…There are interesting results obtained with constituent quark models [7][8][9][10] as well as with other approaches (see for review [11]). However, the given theoretical description usually works well only on limited Q 2 range.…”

Nucleon electromagnetic form factors

“…The resulting constituent size is around 0.2 -0.3 fm. Using such effective constituent quark FF in the light-front form calculation of [Car00], allows a good description of the individual nucleon FFs, see [Pac00]. Note however that the experimental G Ep /G M p ratio can basically be reproduced using pointlike constituent quarks, see Fig.…”

“…The WFs in the calculations described above were however not constructed from a detailed fit to the baryon spectrum. Cardarelli et al subsequently performed a more "microscopic" light-front calculation [Car95,Car00] where the light-front WF was obtained from a rest frame WF which provided a fit to the spectrum. The rest frame WF was taken from the relativized Capstick-Isgur model [Cap86].…”

Baryons as relativistic three-quark bound states

“…Nucleon form factors have been calculated in light-front constituent-quark models [517][518][519][520][521][522], also including dressed quark form factors which effectively add higher Fock components to the wave functions [523][524][525][526]. Light-front quark model calculations are also available for nucleon resonances [21,527,528].…”