Measurement of the Light Antiquark Flavor Asymmetry in the Nucleon Sea

A precise measurement of the ratio of Drell-Yan yields from an $800\mathrm{GeV}/c$ proton beam incident on hydrogen and deuterium targets is reported. Over 140 000 Drell-Yan muon pairs with dimuon mass ${M}_{{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}}\ensuremath{\ge}4.5\mathrm{GeV}/{c}^{2}$ were recorded. From these data, the ratio of antidown ( $\overline{d}$) to antiup ( $\overline{u}$) quark distributions in the proton sea is determined over a wide range in Bjorken $x$. A strong $x$ dependence is observed in the ratio $\overline{d}/\overline{u}$, showing substantial enhancement of $\overline{d}$ with respect to $\overline{u}$ for $x<0.2$. This result is in fair agreement with recent parton distribution parametrizations of the sea. For $x>0.2$, the observed $\overline{d}/\overline{u}$ ratio is much nearer unity than given by the parametrizations.