Angular distribution of large qsub(T) muon pairs in different reference frames

The angular distribution of large transverse momentum muon pairs produced in hadron-hadron collisions is calculated in lowest order of perturbative QCD. It is shown that for the process quark+antiquark→gluon+μ+μ- the polar and azimuthal angle distributions can be made independent of the parton distributions by choosing a special reference frame, the Collins-Soper frame, but for the process quark+gluon→quark+μ+μ- this is not possible. (Auth.)