The light flavor antiquark distributions of the nucleon sea are calculated in the effective chiral quark model and compared with experimental results. The contributions of the flavor-symmetric seaquark distributions and the nuclear EMC effect are taken into account to obtain the ratio of Drell-Yan cross sections σ pD /2σ pp , which can match well with the results measured in the FermiLab E866/NuSea experiment. The calculated results also match the measuredd(x) −ū(x) from different experiments, but unmatch the behavior ofd(x)/ū(x) derived indirectly from the measurable quantity σ pD /2σ pp by the FermiLab E866/NuSea Collaboration at large x. We suggest to measure againd(x)/ū(x) at large x from precision experiments with careful experimental data treatment. We also propose an alternative procedure for experimental data treatment.

We discuss the isospin symmetry breaking (ISB) of the valence-and sea-quark distributions between the proton and the neutron in the framework of the chiral quark model. We assume that isospin symmetry breaking is the result of mass differences between isospin multiplets and then analyze the effects of isospin symmetry breaking on the Gottfried sum rule and the NuTeV anomaly.We show that, although both flavor asymmetry in the nucleon sea and the ISB between the proton and the neutron can lead to the violation of the Gottfried sum rule, the main contribution is from the flavor asymmetry in the framework of the chiral quark model. We also find that the correction to the NuTeV anomaly is in an opposite direction, so the NuTeV anomaly cannot be removed by isospin symmetry breaking in the chiral quark model. It is remarkable that our results of ISB for both valence-and sea-quark distributions are consistent with the Martin-Roberts-Stirling-Thorne parametrization of quark distributions.