Nuclear dependence of dimuon production at 800 GeV

A precise measurement of the atomic-mass dependence of dimuon production induced by 800-GeV protons is reported. Over 450 000 muon pairs with dimuon mass M\ensuremath{\ge}4 GeV were recorded from targets of $^{2}\mathrm{H}$, C, Ca, Fe, and W. The ratio of dimuon yield per nucleon for nuclei versus $^{2}\mathrm{H}$, R=${\mathit{Y}}_{\mathit{A}}$/${\mathit{Y}}_{2\mathit{H}}$, is sensitive to modifications of the antiquark sea in nuclei. No nuclear dependence of this ratio is observed over the range of target-quark momentum fraction 0.1${\mathit{x}}_{\mathit{t}}$0.3. For ${\mathit{x}}_{\mathit{t}}$0.1 the ratio is slightly less than unity for the heavy nuclei. These results are compared with predictions of models of the European Muon Collaboration effect.