  We measure proton structure parameters sensitive primarily to valence quarks
using 8.6 fb$^{-1}$ of data collected by the D0 detector in $\sqrt{s}=1.96$ TeV
$p\bar{p}$ collisions at the Fermilab Tevatron. We exploit the property of the
forward-backward asymmetry in dilepton events to be factorized in to distinct
structure parameters and electroweak quark-level asymmetries. Contributions to
the asymmetry from $s$, $c$ and $b$ quarks, as well as from $u$ and $d$ quarks,
are suppressed allowing valence $u$ and $d$ quarks to be separately determined.
We find and $u$ to $d$ quark ratio near the peak values in the quark density
distributions that is smaller than predictions from modern parton distribution
functions.
