Is theqq¯sea in the nucleon isospin symmetric?

The New Muon Collaboration (NMC) has recently performed a detailed analysis of the difference between the proton and neutron structure functions. They found that the Gottfried sum $\int \mathrm{dx}\frac{\left[F_2^p\mathrm{}\right(x)-F_2^n\mathrm{}(x\left)\right]}{x}$ is significantly below the simple quark-parton model prediction of $\frac{1}{3}$, which indicates a sizable isospin asymmetry of the $q\overline{q}$ sea in the nucleon. We have carried out a theoretical analysis of the Gottfried sum on the basis of a chiral-quark-soliton model, and obtained a satisfactory agreement with the NMC measurement.