Angular distributions of hadron produced lepton pairs

We measured the angular distribution parameters of lepton pairs produced by high-energy negative pions on deuterium and tungsten targets. Their dependence on the transverse momentum PT of the pair is found to be at variance with recent predictions based on the soft-gluon-emission (SGE) approximation to perturbative QCD. A relation between two of the parameters, analogous to the Callan-Gross relation in deep inelastic scattering, is found to be not satisfied. Our statistics at large relative momentum x1 of the parton in the target hadron, are insufficient to substantiate the higher-twist prediction. (orig.)