Measurement of the Light Antiquark Flavor Asymmetry in the Nucleon Sea

TL;DR

This paper reports a precise measurement of the anti-down to anti-up quark ratio in the proton sea, revealing a strong x dependence and an enhancement of dbar over ubar at low x, which informs our understanding of nucleon structure.

Contribution

It provides the first detailed measurement of the light antiquark flavor asymmetry in the nucleon sea over a wide x range, challenging existing parton distribution models at higher x.

Findings

Significant dbar/ubar asymmetry observed for x<0.2.

dbar/ubar ratio approaches unity for x>0.2.

Results align with some models but challenge others at high x.

Abstract

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