Angular Distributions in the Drell-Yan Process: A Closer Look at Higher Twist Effects

We calculate the angular distribution of the lepton produced in the Drell-Yan reaction taking into account pion bound state effects. This is particularly interesting since the standard perturbative QCD prediction is inconsistent with the data. We work in the kinematic region where one of the pion constituents goes far off shell, which allows us to treat the bound state problem perturbatively. We show that the angular distribution is very sensitive to the shape of the pion distribution amplitude. The model we discuss fits the data if we choose a two-humped pion distribution amplitude suggested by QCD sum rules.