Measuring the flavor asymmetry in the sea quarks of the proton

Abstract

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 of gluons and quark-antiquark pairs. By measuring DrellYan scattering, the Fermilab E-906/SeaQuest 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, E-906/SeaQuest 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 suggests that the antiquark distributions and hence the sea distributions are not modified. To accomplish these goals, the experiment will used 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. The collaboration expects to begin data collection in fall 2010.