Parton distribution functions and quark orbital motion

A covariant version of the quark-parton model is studied. The dependence of the structure functions and parton distributions on the 3D intrinsic motion of the quarks is discussed. The important role of the orbital momentum of the quark, which is a particular case of intrinsic motion, appears as a direct consequence of the covariant description. The effect of the orbital motion is substantial, especially for polarized structure functions. At the same time, the procedure for obtaining the momentum distributions of polarized quarks from the combination of polarized and unpolarized structure functions is suggested.