Double-spin asymmetries for small-Q_T Drell-Yan pair production in transversely polarized pbar{p} collisions

We discuss the Drell-Yan process at a measured transverse-momentum $Q_T$ of the produced lepton pair in collisions of transversely polarized protons and antiprotons, to be observed at the proposed spin experiments at GSI. The large logarithmic contributions from multiple soft gluon emission, accompanying the Drell-Yan mechanism at small $Q_T$, are resummed to all orders in QCD perturbation theory up to next-to-leading logarithmic (NLL) accuracy. Numerical evaluation shows the impact of the NLL as well as LL effect on the dilepton $Q_T$ spectra. For the corresponding $Q_T$-dependent spin asymmetry $\aqt$, the LL effect gives significant modification while the NLL effect is marginal, leading to QCD prediction that $\aqt$ at GSI is flat at small and moderate $Q_T$ and almost equals the conventional asymmetry $A_{TT}$ associated with the $Q_T$-integrated cross sections. This flat behavior in turn allows us to use analytic saddle-point evaluation of the resummation formula in the limit $Q_T\to 0$, not only to obtain quantitative estimate of $\aqt$, but also to clarify mechanisms behind the relation $\aqt \simeq A_{TT}$ characteristic of $p\bar{p}$ collisions at GSI.