The QCD Parton Model: a Useful Approximation

Approximate relations among transverse momentum dependent quark distribution functions are established in the framework of the QCD parton model. The validity of such results survives QCD evolution effects, owing to the Politzer theorem on equations of motion. Furthermore the model fixes an energy scale, involved in the parametrization of the correlator, which determines the Q^2 dependence of the azimuthal asymmetry in inclusive reactions. Some of the present data - the cos2\phi symmetry in unpolarized Drell-Yan, the sin2\phi single spin asymmetry in semi-inclusive deep inelastic scattering (SIDIS) and the cos\phi asymmetry in unpolarized SIDIS - support model predictions. Further measurements of SIDIS and Drell-Yan asymmetries are suggested, in particular the double spin asymmetry in SIDIS, which allows to determine approximately the proton transversity.