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

Abstract

The neutron charge form factor GEn(Q**2) 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(Q**2)$ receives a relativistic correction which cancels exactly against the Foldy term in GEn(Q**2), 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(Q**2) / GMn(Q**2) 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(Q**2) =/ 0 and GMp(Q**2)/GMn(Q**2) =/ -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(Q**2)$ and GMp(Q**2)/GMn(Q**2).