Opportunities with Drell-Yan scattering: Probing sea quarks in the nucleon and nuclei

The large (d) over bar (x)/(u) over bar (x) ratio observed by Fermilab E866/NuSea convincingly demonstrated that the sea is not simply a result of pQCD. Moreover, meson cloud models also failed to explain fully the observed kinematic dependence. The Drell-Yan mechanism offers a unique, selective probe of antiquarks in the nucleon. Fermilab has approved a new Drell-Yan experiment, E906, that will exploit this feature to probe (d) over bar (x)/(u) over bar (x) by measuring the ratio of cross-sections for the proton-induced Drell-Yan process on hydrogen to deuterium. When the nucleon is contained in a nucleus, the nucleon's parton distributions should to be modified; although this effect was not seen in the sea quark distributions obtained by Fermilab E772 with Drell-Yan scattering. The upcoming E906 Drell-Yan experiment will provide much more precise measurements over a wider kinematic range in order to guide and challenge the theoretical models.