Quark asymmetries in the proton from a model for parton densities

Based on quantum fluctuations in momentum and of the proton into meson-baryon pairs, we develop a physical model for the nonperturbative x shape of parton density functions in the proton. The model describes the proton structure function and gives a natural explanation of observed quark asymmetries, such as the difference between the anti-up and anti-down sea quark distributions and between the up and down valence distributions. An asymmetry in the momentum distribution of strange and antistrange quarks in the nucleon is found to reduce the NuTeV anomaly to a level which does not give a significant indication of physics beyond the standard model.