A Tale of Two Protons, or two tales of the Proton’s sea

Paul Reimer, Argonne National Lab

The conventional picture of the proton is based on three 'valence' quarks—two 'up' and one 'down'. This picture has done a remarkable job of describing many properties of the proton. However, thanks to the richness of QCD, the proton is a much more complicated object. In addition to the valence quarks, the proton contains a “sea” of quark-antiquark pairs and gluons that bind the system together. Experiments have been able to explore and map the sea quark distributions, but reconciling this data with a model for the origin of the sea is difficult. The Drell-Yan mechanism provides a tool that can select only the antiquark distributions in the target, making it an ideal process with which to study the sea. Using Drell-Yan, a remarkable asymmetry has been observed in the difference of anti-down to anti-up quarks in the proton. This asymmetry cannot simply be generated through perturbative QCD, but rather indicates an underlying and fundamental antiquark component in the proton. The Fermilab E-906 Drell-Yan experiment will extend earlier measurements of this difference to larger values of xBj and increase statistical precision at lower xBj. To accomplish this goal, a reconfiguration of earlier Drell-Yan experiments is underway. The collaboration expects to begin data collection summer, 2010.