QCD Selection Rules in Polarized Hadron Collisions
R. L. Jaffe, N. Saito
TL;DR
The paper investigates spin asymmetries in polarized hadron collisions to test perturbative QCD selection rules. It clarifies the leading-twist helicity, chirality, and transversity structure of parton distributions and explains why gluon-induced transverse spin effects are suppressed relative to longitudinal ones in jet and direct-photon production, while Drell–Yan can yield comparable transverse and longitudinal asymmetries. Using twist-two distributions and three gluon-polarization models, it provides numerical estimates for RHIC-scale collisions with Pythia and GS95 PDFs, assuming $\delta q \approx \Delta q$. The results show ${\cal A}_{TT}$ at most a few $\times10^{-3}$ and ${\cal A}_{TT}/{\cal A}_{LL}$ typically well below unity, offering a precise benchmark: any observed nonzero transverse asymmetry would challenge the standard spin-structure picture of perturbative QCD.
Abstract
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.