Evidence for a flavor asymmetry between the uˉ and dˉ quark
distributions in the proton has been found in deep-inelastic scattering and
Drell-Yan experiments. The pronounced dependence of this flavor asymmetry on
x (fraction of nucleon momentum carried by partons) observed in the Fermilab
E866 Drell-Yan experiment suggested a drop of the dˉ(x)/uˉ(x) ratio in the x>0.15 region. We report results from the
SeaQuest Fermilab E906 experiment with improved statistical precision for dˉ(x)/uˉ(x) in the large x region up to x=0.45
using the 120 GeV proton beam. Two different methods for extracting the
Drell-Yan cross section ratios, σpd/2σpp, from the SeaQuest
data give consistent results. The dˉ(x)/uˉ(x) ratios and the dˉ(x)−uˉ(x) differences are deduced from these cross section ratios for
0.13<x<0.45. The SeaQuest and E866/NuSea dˉ(x)/uˉ(x) ratios are in good agreement for the x≲0.25
region. The new SeaQuest data, however, show that dˉ(x)
continues to be greater than uˉ(x) up to the highest x value
(x=0.45). The new results on dˉ(x)/uˉ(x) and dˉ(x)−uˉ(x) are
compared with various parton distribution functions and theoretical
calculations