Up and Down Quark Structure of the Proton

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.