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 \leq x_\scriptscriptstyle\rm B \leq 0.83, where x_\scriptscriptstyle\rm 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_\scriptscriptstyle\rm 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: \left. d_v/u_v \right|_x_\scriptscriptstyle\rm B\to 1 = 0.230 (57). Requiring consistency with this result presents a challenge to many descriptions of proton structure.