d/u Asymmetry in the Nucleon Sea

Fermilab E866 has performed a precise measurement of the ratio of Drell-Yan yields from an 800 GeV/c proton beam incident on hydrogen and deuterium targets, leading to the first determinations of bar d/bar u and bar d-bar u in the proton as functions of χ. The results show that bar d > bar u over a broad range of χ and provide valuable information regarding the origins of the bar d/bar u asymmetry and the antiquark sea in the nucleon. No known symmetry requires equality of the bar d and bar u distributions in the proton. However, until recently it had been generally assumed that bar d(χ) ∼ bar u(χ), where χ is the fraction of the proton's momentum (Bjorken-χ) carried by the antiquark, based both on the assumption that the majority of the antiquark sea in the nucleon originates from gluon splitting into q - bar q pairs and the lack of experimental evidence to the contrary. The first clear evidence that bar d ≠ bar u came from the NMC measurements of the structure functions F2p(χ) and F2n(χ) in deep-inelastic muon scattering on hydrogen and deuterium