Measurement of flavor asymmetry of light-quark sea in the proton with Drell-Yan dimuon production in $p+p$ and $p+d$ collisions at 120 GeV

Evidence for a flavor asymmetry between the $\bar{u}$ and $\bar{d}$ 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 $\bar{d}(x)/\bar{u}(x)$ ratio in the $x>0.15$ region. We report results from the SeaQuest Fermilab E906 experiment with improved statistical precision for $\bar{d}(x)/\bar{u}(x)$ 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, $\sigma^{pd}/2\sigma^{pp}$, from the SeaQuest data give consistent results. The $\bar{d}(x)/\bar{u}(x)$ ratios and the $\bar{d}(x)-\bar{u}(x)$ differences are deduced from these cross section ratios for $0.13<x<0.45$. The SeaQuest and E866/NuSea $\bar{d}(x)/\bar{u}(x)$ ratios are in good agreement for the $x\lesssim 0.25$ region. The new SeaQuest data, however, show that $\bar{d}(x)$ continues to be greater than $\bar{u}(x)$ up to the highest $x$ value ($x=0.45$). The new results on $\bar{d}(x)/\bar{u}(x)$ and $\bar{d}(x)-\bar{u}(x)$ are compared with various parton distribution functions and theoretical calculations.