Dilepton production near partonic threshold in transversely polarized proton-antiproton collisions
Hirotaka Shimizu, George Sterman, Werner Vogelsang, Hiroshi Yokoya
TL;DR
The paper investigates dilepton (Drell–Yan) production in transversely polarized $\bar{p}p$ collisions near partonic threshold at GSI, focusing on extracting transversity densities. It analyzes higher-order QCD corrections using fixed-order results and all-order soft-gluon threshold resummation (NLL), including an infrared cutoff model to regulate far-infrared effects. The study finds large perturbative corrections to both unpolarized and transversely polarized cross sections due to soft-gluon emission, while the double-spin asymmetry $A_{TT}$ remains comparatively robust to these corrections and cutoffs. The results suggest unpolarized cross sections in this regime can illuminate the interplay between perturbative and nonperturbative QCD dynamics, whereas $A_{TT}$ provides a reliable observable for transversity in GSI kinematics.
Abstract
It has recently been suggested that collisions of transversely polarized protons and antiprotons at the GSI could be used to determine the nucleon's transversity densities from measurements of the double-spin asymmetry for the Drell-Yan process. We analyze the role of higher-order perturbative QCD corrections in this kinematic regime, in terms of the available fixed-order contributions as well as of all-order soft-gluon resummations. We find that the combined perturbative corrections to the individual unpolarized and transversely polarized cross sections are large. We trace these large enhancements to soft gluon emission near partonic threshold, and we suggest that with a physically-motivated cut-off enhancements beyond lowest order are moderated relative to resummed perturbation theory, but still significant. The unpolarized dilepton cross section for the GSI kinematics may therefore provide information on the relation of perturbative and nonperturbative dynamics in hadronic scattering. The spin asymmetry turns out to be rather robust, relatively insensitive to higher orders, resummation, and the cut-offs.