Energy loss of fast quarks in nuclei
  We report an analysis of the nuclear dependence of the yield of Drell-Yan dimuons from the 800 GeV/c proton bombardment of $^2H$, 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 $-dE/dz = 2.32 \pm 0.52\pm 0.5$ GeV/fm. This is the first observation of a nonzero energy loss of partons traveling in nuclear environment. 
M.B. Johnson, B.Z. Kopeliovich, I.K. Potashnikova, P.L. McGaughey,   J.M. Moss, J.C.Peng, G.T.Garvey, M.J. Leitch, M.R. Adam...