Probing the Large-$x$ Structure of the Proton with Drell-Yan Scattering at Fermilab

The Drell-Yan reaction can be used to probe the parton distribution of the interaction hadrons. In a fixed target environment, where the detector acceptance selects only large-$x_F$ events, Drell-Yan probes the high-$x$ (Bjorken-$x$) valence distributions of the beam and the low- to intermediate-$x$ sea antiquark distributions of the target. Fermilab E866/NuSea has measured the Drell-Yan absolute cross section of 800 GeV/c protons on hydrogen and deuterium targets. These data are the first measurements of the Drell-Yan cross section in $pp$ collisions over a broad kinematic region and the most extensive study to date of the Drell-Yan cross section in $pd$ collisions. The absolute cross sections, with full radiative corrections, will be presented and comparisons made with with next-to-leading order calculations based on modern parton distributions. The results show that recent global parton distribution fits provide a good description of the light antiquark sea in the nucleon over the range $0.03 < x_\textrm{target} < 0.15$. When projected against $x_\textrm{beam}$ the data indicate that most modern parton distributions appear to overestimate the valence parton distributions at large-$x$. We plan to extend these studies in Fermilab E906, which will use a 120 GeV/c proton beam from the Fermilab Main Injector to collect better statistics at even larger values of $x_\textrm{beam}$.