Relativistic effects on the neutron charge form factor in the constituent quark model

The neutron charge form factor GEn(Q2) is investigated within a constituent quark model formulated on the light-front. It is shown that, if the quark initial motion is neglected in the Melosh rotations, the Dirac neutron form factor F1n(Q2) receives a relativistic correction which cancels exactly against the Foldy term in GEn(Q2), as it has been recently argued by Isgur. Moreover, at the same level of approximation the ratio of the proton to neutron magnetic form factors GMp(Q2)/GMn(Q2) is still given by the naive SU(6)-symmetry expectation, −3/2. However, it is also shown that the full Melosh rotations break SU(6) symmetry, giving rise to GEn(Q2)≠0 and GMp(Q2)/GMn(Q2)≠−3/2 even when a SU(6)-symmetric canonical wave function is assumed. It turns out that relativistic effects alone cannot explain simultaneously the experimental data on GEn(Q2) and GMp(Q2)/GMn(Q2).