Coherence time in high energy proton-nucleus collisions

Precisely measured Drell-Yan cross sections for 800thinspthinspGeV protons incident on a variety of nuclear targets exhibit a deviation from linear scaling in the atomic number A . We show that this deviation can be accounted for by energy degradation of the proton as it passes through the nucleus if account is taken of the time delay of particle production due to quantum coherence. We infer an average proper coherence time of 0.4{plus_minus}0.1 fm/c , corresponding to a coherence path length of 8{plus_minus}2 thinspthinspfm in the rest frame of the nucleus. {copyright} {ital 1999} {ital The American Physical Society }