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Measurements of the ratio of Drell-Yan yields from an 800 GeV/c proton beam incident on liquid hydrogen and deuterium targets are reported. Approximately 360,000 Drell-Yan muon pairs remained after all cuts on the data. ¿From these data, the ratio of anti-down (d) to anti-up (ū) quark distributions in the proton sea is determined over a wide range in Bjorken-x. These results confirm previous measurements by E866 and extend them to lower x. From these data, (d −ū) and (d −ū)dx are evaluated for 0.015 < x < 0.35. These results are compared with parameterizations of various parton distribution functions, models and experimental results from NA51, NMC and HERMES. 13.85.Qk; 14.20.Dh; 24.85.+p; 14.65.Bt

A precise measurement of the ratios of the Drell-Yan cross section per nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is reported. The behavior of the Drell-Yan ratios at small target parton momentum fraction is well described by an existing fit to the shadowing observed in deep-inelastic scattering. The cross section ratios as a function of the incident parton momentum fraction set tight limits on the energy loss of quarks passing through a cold nucleus. The Drell-Yan process, where a beam quark (antiquark) fuses with a target antiquark (quark) producing a muon pair, can be used to study the interactions of fast partons penetrating through cold nuclei. Only initial state interactions are important in Drell-Yan since the dimuon in the final state does not interact strongly with the partons in the medium. This makes Drell-Yan scattering an ideal tool to study energy loss of fast quarks in nuclear matter by comparing the observed yields from a range of nuclear targets. The dynamics of fast parton energy loss in nuclear matter is the subject of considerable theoretical interest [1][2][3][4] and has significant implications for the physics of relativisitic heavy ion collisions.Drell-Yan scattering is closely related to deep-inelastic scattering (DIS) of leptons, but unlike DIS it can be used specifically to probe antiquark contributions in target parton distributions. When DIS on nuclei occurs at x < 0.08, where x is the parton momentum fraction, the cross section per nucleon decreases with increasing nuclear number A due to shadowing [5,6].