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

Abstract

The neutron charge form factor G E n ( 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 F 1 n ( Q 2) receives a relativistic correction which cancels exactly against the Foldy term in G E n ( 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 G M p ( Q 2)/ G M n ( 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 G E n ( Q 2)≠0 and G M p ( Q 2)/ G M n ( 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 G E n ( Q 2) and G M p ( Q 2)/ G M n ( Q 2).