SU(6)-breaking symmetry and the ratio of proton momentum distributions

The ratio between the anomalous magnetic moments of proton and neutron has recently been suggested to be connected to the ratio of proton momentum fractions carried by valence quarks. This relation has been obtained within a parametrization of the Generalized Parton Distributions (GPD) \cite{gpv}, but it is completely independent of such a parametrization.\\ It will be shown that using different CQMs this relation holds within a few percent accuracy. This agreement is based on what all the CQMs have in common: the effective degrees of freedom of the three constituent quarks and the underlying SU(6) symmetry.\\ On the other hand, the experimental value of the ratio is not reproduced by CQMs. This means that the SU(6)-breaking mechanism contained in the phenomenological partonic distributions does not correspond to the SU(6) breaking mechanism implemented in the CQMs we have analyzed \cite{noi03}.\\ We will also show how this relation can be used in order to understand in which way to implement an $SU(6)$-breaking mechanism and to test models.