Difference between the u ˉ and d ˉ sea quark distributions in the proton was first observed in the violation of the Gottfried sum rule in deep-inelastic scattering (DIS) experiments. The parton momentum fraction x dependence of this difference has been measured over the region 0.02 < x < 0.35 from Drell–Yan and semi-inclusive DIS experiments. The Drell–Yan data suggested a possible sign-change for d ˉ ( x ) ? u ˉ ( x ) near x ～ 0.3 , which has not yet been explained by existing theoretical models. We present an independent evidence for the d ˉ ( x ) ? u ˉ ( x ) sign-change at x ～ 0.3 from an analysis of the DIS data. We further discuss the x -dependence of d ˉ ? u ˉ in the context of meson cloud model and the lattice QCD formulation.
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