HIGHER-TWIST EFFECTS IN THE DRELL-YAN ANGULAR-DISTRIBUTION

We study the Drell-Yan process piN --> mu+mu-X at large x(F) using per turbative QCD. A higher-twist mechanism suggested by Berger and Brodsk y is known to qualitatively explain the observed x(F) dependence of th e muon angular distribution, but the predicted large x(F) behavior dif fers quantitatively from observations. We have repeated the model calc ulation taking into account the effects of nonasymptotic kinematics. A t fixed-target energies we find important corrections which improve th e agreement with data. The asymptotic result of Berger and Brodsky is recovered only at much higher energies. We discuss the generic reasons for the large corrections at high x(F). A proper understanding of the x(F) --> 1 data would give important information on the pion distribu tion amplitude and exclusive form factor.