L

We report an analysis of the nuclear dependence of the yield of Drell-Yan dimuons from the 800 GeV/c proton bombardment of 2 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 ± 0.52 ± 0.5 GeV/fm. This is the first observation of a nonzero energy loss of partons traveling in nuclear environment.24.85.+p; 13.85.Qk; 25.40.Ve For many years it has been suggested that quark energy loss might give rise to a nuclear dependence [1][2][3][4] of the cross section of Drell-Yan [5] (DY) production. When a proton enters a nucleus the first (soft) inelastic collision liberates a quark, which then loses energy via hadronization (due to confinement) and interaction in the nuclear medium. A lepton pair created from a subsequent interaction then has reduced energy compared with the DY process on a free nucleon. The goal of the present analysis is to search for this effect in the nuclear dependence of the DY process.Fermilab E772 made a precise measurement of the nuclear dependence of the DY process using 800 GeV/c protons. The experimental details of E772 have been described previously [6][7][8]. Briefly we indicate those germane to the present discussion. Muon pairs were recorded from targets of 2 H, C, Ca, Fe, and W, in the mass range M ≥ 4 GeV/c 2 . Excluding the Υ resonance region, 9 ≤ M ≤ 11 GeV/c 2 , we reconstruct 2.5 × 10 5

Absolute cross sections as functions of kinematic variables are presented for the production of muon pairs from 800 GeV proton bombardment of 'H. Drell-Yan (continuum) dimuons were recorded in the mass regions 4.5 5 M,+,-5 9 GeV and M , -, 2 11 GeV, with an x-Feynman range -0.1 5 X F < 0.75. This range corresponds to smaller masses and larger values of X F than previous 800 GeV Drell-Yan data. Cross sections for the T (IS) resonance are also given versus the transverse momentum and X F . PACS number(s): 13.85.Qk, 12.38.Qk, 25.40.Ve

No abstract