D0 Collab.
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 √s=1.96 TeV pp¯ 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.