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Anomalous Evolution of Nonsinglet Nucleon Structure Functions

We calculate the scale dependence of nonsinglet nucleon structure functions. Due to anomalous axial symmetry breaking a large flavour asymmetry of the quark--antiquark sea is generated nonperturbatively. This produces a strong scale dependence of the nonsinglet structure function in an intermediate range of $Q^2$. Evolving nonperturbatively a pure valence distribution from an infrared scale we can thus compute $F_2^p-F_2^n$ as measured by the NMC, and give detailed predictions for its $Q^2$ dependence at fixed $x$. We also compare our results with Drell--Yan data.