Angular distributions of muon pairs produced by negative pions on deuterium and tungsten

We present the angular distributions of high-mass muon pairs produced in a high-statistics experiment by 140 and 194 GeV/c π^- beams impinging on a tungsten target, and by 286 GeV/c π^- beam on deuterium and tungsten targets. We find no evidence for a center-of-mass energy dependence or a nuclear dependence of the angular distribution parameters. The two parameters λ and μ are found to be essentially independent of any kinematical variable. In contrast, the parameter v increase with the dimuon transverse momentum P _T , at variance with recent perturbative QCD predictions. Our statistics at large x ₁ are insufficient to substantiate the highertwist prediction.