The x-Dependent Helicity Distributions for Valence Quarks in Nucleons
  We derive simple relations between the polarized and unpolarized valence quark distributions in a light-cone SU(6) quark-spectator model for nucleons. The explicit x-dependent Wigner rotation effect for the light-flavor quarks is calculated. It is shown that the mass difference between the scalar and vector spectators could reproduce the up and down valence quark asymmetry that accounts for the observed ratio $F_2^{n}/F_2^{p}$. The proton, neutron, and deuteron polarization asymmetries, $A_1^p$, $A_1^n$, and $A_1^d$, are in agreement with the available data by taking into account the Wigner rotation effect. The calculated $x$-dependent helicity distributions of the up and down valence quarks are compared with the recent results from semi-inclusive hadron asymmetries in polarized deep inelastic scattering by the Spin Muon Collaboration. 
Bo-Qiang Ma (IHEP, Academia Sinica, Beijing)