The range of validity of the QCD-improved parton model

Based on the world DIS data we extract the experimental F2p−F2n as a function of Bjorken-x and photon virtuality Q2, using two different methods. Both methods lead to identical results. We find that the standard PDFs fail to describe the experimental data below Q2< 7 GeV2, which is much higher than for F2p and F2d separately. The difference between PDFs and the experimental data cannot be understood as due to nuclear effects in the deuteron, and evidently suggests substantial nonsinglet higher-twist effects. The trend of the experimental data is approximately explained by a recent two-component model of the nucleon structure functions and suggests strong Q2-dependence of the Gottfried Sum Rule, in disagreement with the parton model interpretation. The large negative higher-twist effects can explain the difference between the value of the Gottfried integral obtained recently by the E866 Drell-Yan experiment at Fermilab and an older NMC result.