F. Cardarelli et S. Simula, SU(6) breaking effects in the nucleon elastic electromagnetic form factors- art. no. 065201, PHYS REV C, 6206(6), 2000, pp. 5201
The effects of both kinematical and dynamical SU(6) breaking on the nucleon
elastic form factors, G(E)(N)(Q(2)) and G(M)(N)(Q(2)), are investigated wi
thin the constituent quark model formulated on the light front. The investi
gation is focused on G(E)(n)(Q(2)) and the ratio G(M)(p)(Q(2))/G(M)(n)(Q(2)
), which within the SU(6) symmetry are given by; G(E)(n)(Q(2))=0 and G(M)(p
)(Q(2))/G(M)(n)(Q(2))=-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-quar
k interaction, can affect both G(E)(n)(Q(2)) and G(M)(p)(Q(2))/G(M)(n)(Q(2)
). The calculated G(E)(n)(Q(2)) is found to be qualitatively consistent wit
h existing data, though only similar or equal to 65% of the experimental ne
utron 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)(Q(2))/G(M)(n)
(Q(2)) turn out to be inconsistent with the data. It is, however, shown tha
t the calculations of G(M)(N)(Q(2)) based on different components of the on
e-body electromagnetic current lead to quite different results. In particul
ar, 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 l
ight-front formalism. These unwanted effects can be avoided by considering
the transverse y component of the current. In this way our light-front pred
ictions are found to be consistent with the data on both G(E)(n)(Q(2)) and
G(M)(p)(Q(2))/G(M)(n)(Q(2)). Finally, it is shown that a suppression of the
ratio G(E)(p)(Q(2))/G(M)(p)(Q(2)) With respect to the dipole-fit predictio
n can be expected in the constituent quark model provided the relativistic
effects generated by the Melosh rotations of the constituent spins are take
n into account.