For massive-lepton pair production in meson-induced reactions, we use quantum chromodynamics perturbation theory to predict that the decay angular distribution in the pair rest frame will change from predominantly $1+{cos}^2\theta$ to ${sin}^2\theta$ as the longitudinal-momentum fraction of the pair $x_{\mathrm{F}}\to +1\mathrm{}$. The two angular distributions are associated respectively with ${\mathrm{}(1-x)\mathrm{}}^2$ and $Q^{-2\mathrm{}}{\mathrm{}(1-x)\mathrm{}}^0$ components of the valence-quark structure function of the meson.

• Received 18 January 1979

DOI:https://doi.org/10.1103/PhysRevLett.42.940