Measurement of the Light Antiquark Flavor Asymmetry in the Nucleon Sea

A precise measurement of the ratio of Drell-Yan yields from an 800GeV/c proton beam incident on hydrogen and deuterium targets is reported. Over 140 000 Drell-Yan muon pairs with dimuon mass M_μ⁺μ^-≥4.5GeV/c² were recorded. From these data, the ratio of antidown ( d̄) to antiup ( ū) quark distributions in the proton sea is determined over a wide range in Bjorken x. A strong x dependence is observed in the ratio d̄/ū, showing substantial enhancement of d̄ with respect to ū for $x<0.2$. This result is in fair agreement with recent parton distribution parametrizations of the sea. For $x>0.2$, the observed d̄/ū ratio is much nearer unity than given by the parametrizations.