QCD Selection Rules in Polarized Hadron Collisions
  Plans are underway to measure spin asymmetries at large momentum transfer in hadron hadron collisions at RHIC and elsewhere. Proposals have focused on measuring quark transversity and quark and gluon helicity distributions in the nucleon. These experiments will also provide a strong and simple test of perturbative QCD, namely that ${\cal A}_{TT}/{\cal A}_{LL}\ll 1$ in $pp\rightarrow 2\, {\rm jets} +X$ and several related processes, whereas ${\cal A}_{TT}/{\cal A}_{LL}\sim 1$ in Drell-Yan production of muon pairs. The prediction tests the helicity, twist and chirality selection rules of perturbative QCD that form the foundation for the analysis of spin dependent hard processes. We estimate the ratio ${\cal A}_{TT}/{\cal A}_{LL}$ numerically for polarized protons at RHIC. 
R. L. Jaffe and N. Saito