Measuring the flavor asymmetry in the sea quarks of the proton with the Drell-Yan process

The proton is a composite object made of fundamental, strongly interacting quarks. Many of the features of the proton can be described by a simple picture based on three “valence” quarks bound by the exchange of gluons. However, protons are much more complex objects with the vast majority of their mass dynamically generated by quantum chromodynamics (QCD). This mass manifests itself through a “sea” of gluons and quark-antiquark pairs. By measuring Drell-Yan scattering, the Fermilab E-906/Drell-Yan experiment will study the sea quark distribution in the proton and, in particular, the unusually large asymmetry between anti-up and anti-down quarks measured by earlier Drell-Yan experiments. This asymmetry cannot simply be generated through pair creation, but rather indicates an underlying, fundamental antiquark component in the proton. Using the same technique, the E-906/SeaQuest experiment will also investigate the differences between the antiquark distributions of the free proton and a proton bound in a nucleus. Nuclear binding is expected to modify the quark distributions and it has long been known that the overall quark distributions are different (the EMC effect). Surprisingly, present data suggest, however, that the antiquark distributions are not modified. To accomplish these goals, the experiment will use a 120 GeV proton beam extracted from the Fermilab Main Injector. While the experiment will be taking advantage of equipment from earlier Drell-Yan experiments, the changes in kinematics of the experiment require several, significant upgrades to the spectrometer. Japanese institutions, including Tokyo Institute of Technology, KEK, RIKEN, Kyoto University and Yamagata University, are making substantial contributions to this upgrade. The collaboration expects to begin data collection in Summer, 2010.