  We have developed a physical model for the non-perturbative x-shape of parton
density functions in the proton, based on Gaussian fluctuations in momenta, and
quantum fluctuations of the proton into meson-baryon pairs. 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. We find an
asymmetry in the momentum distribution of strange and anti-strange quarks in
the nucleon, large enough to reduce the NuTeV anomaly to a level which does not
give a significant indication of physics beyond the standard model. We also
consider charmed fluctuations, and show that they can explain the excess at
large x in the EMC F2c data.
