A Non-perturbative Model of Mean Field Quarks*

We develop a non-perturbative model for valence quark distributions in the nucleon to account for the phenomena related to quark interactions in the mean field of the nucleon interior.  The main motivation for the model is to obtain a mean field description of the valence quarks as a baseline upon which short range quark-quark interactions generate the high x tail of valence quark distributions. The model is based on the pionic cloud picture of the nucleon in which one assumes a factorization of the short-range valence quark and long range pion-cloud dynamics. The nucleon structure function is calculated using light-front dynamics and the effective Feynman diagrammatic approach is used to introduce valence quark and pion-cloud wave functions. The parameters of these wave functions are fixed by the position of the peak of xf_q(x) distribution of valence quarks at Q_0, corresponding to the charm-quark mass. With few parameters we achieved a very reasonable description of the up and down valence quark distributions in the moderate x region (x< 0.5) where one expects the mean field dynamics to dominate. The model however systematically underestimates the high x region, where we expect enhanced contribution from partonic correlations.