Single Transverse-Spin Asymmetries in Hadronic Pion Production

TL;DR

The paper addresses the origin of large single transverse-spin asymmetries in inclusive hadronic pion production by applying twist-3 QCD factorization. It identifies the dominant forward-region contributions as arising from a single twist-3 quark-gluon correlator, $T^{(V)}_F(x,x)$, and shows that its derivative drives the asymmetry, enabling calculable predictions when paired with standard twists-2 PDFs and fragmentation functions. Using a simple model $T^{(V)}_F(x,x) = \kappa_a \lambda q_a(x)$, the authors reproduce Fermilab data for $\pi^+$, $\pi^-$, and $\pi^0$, extract a normalization $\lambda \sim 0.07$–$0.08$ GeV, and make predictions for RHIC, with a notably mild $\ell_T$-dependence in the forward regime. The work demonstrates that perturbative QCD can describe spin phenomena at moderate transverse momenta and provides a framework for probing twist-3 distributions and their universality across processes and energies.

Abstract

We analyze single transverse-spin asymmetries for hadronic pion production at large transverse momenta using QCD factorization. In the large $x_F$ region, leading contributions to the asymmetries are naturally produced by twist-3 parton correlation functions that couple quark fields and gluon field strengths. With a simple model for these matrix elements, leading-order asymmetries calculated from QCD are consistent with data on pion production from Fermilab, and can be used to predict single-spin asymmetries at RHIC. We argue that our perturbative calculation for the asymmetries is relevant to pion transverse momenta as low as a few GeV.

Figures (29)

• Figure 1: Inclusive lepton-hadron deep-inelastic scattering, with the target hadron polarized transversely.
• Figure 2: Quark-quark scattering diagrams that give a nonvanishing single transverse-spin asymmetry in large-$p_T$ reactions AN:KPR.
• Figure 3: Sketch of single pion production in spin-averaged hadron-hadron collisions.
• Figure 4: Factorization of a typical forward scattering amplitude contributing to the spin-dependent cross section for hadronic pion production: (a) with chiral-even three-parton matrix element, (b) with chiral-odd transversity.
• Figure 5: General Feynman diagrams contributing to the partonic parts $H$ in Eq. (\ref{['s3e11']}).