Exploring the Antiquark Structure of Matter with Drell-Yan Scattering: Fermilab E-906/SeaQuest

Abstract

Deep inelastic scattering (DIS) is an excellent tool for probing the quark structure of hadrons. It has been used to determine precisely the distributions of quarks and to determine the spin they carry. DIS is only sensitive to the charge-squared weight sum of all (valence and sea) quark and cannot easily probe just the sea quark distributions. The Drell-Yan mechanism provides a complementary approach that can select only the antiquark distributions in the target. A remarkable asymmetry has been observed in the difference of anti-down to anti-up quarks in the proton, which 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. Using the same technique, E-906/SeaQuest will investigate the differences between the antiquark distributions of a free and bound proton. Nuclear binding should modify the quark distributions and DIS data has shown that the overall quark distributions are different (the EMC effect). Surprisingly, present data suggests 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. The lower beam energy requires several upgrades and configuration changes to the spectrometer which are underway. The collaboration expects to begin data collection summer, 2010.