We report an analysis of the nuclear dependence of the yield of Drell-Yan dimuons from the $800\mathrm{GeV}/c$ proton bombardment of ${}^2\mathrm{H}$, C, Ca, Fe, and W targets. Employing a new formulation of the Drell-Yan process in the rest frame of the nucleus, this analysis examines the effect of initial-state energy loss and shadowing on the nuclear-dependence ratios versus the incident proton's momentum fraction and dimuon effective mass. The resulting energy loss per unit path length is $-dE/dz=2.32\mathrm{}\pm 0.52\pm 0.5\mathrm{GeV}/\mathrm{fm}$. This is the first observation of a nonzero energy loss of partons traveling in a nuclear environment.

• Received 13 March 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.4483