Valence Quark Ratio in the Proton

Beginning with precise data on the ratio of structure functions in deep inelastic scattering (DIS) from ³He and ³H, collected on the domain 0.19 <= x B <= 0.83, where x B is the Bjorken scaling variable, we employ a robust method for extrapolating such data to arrive at a model-independent result for the x B = 1 value of the ratio of neutron and proton structure functions. Combining this with information obtained in analyses of DIS from nuclei, corrected for target-structure dependence, we arrive at a prediction for the proton valence-quark ratio: dnu /unu q | x B -> 1 = 0.230 (57). Requiring consistency with this result presents a challenge to many descriptions of proton structure.