Twist-3 Single-Spin Asymmetries in Semi-Inclusive Deep-Inelastic Scattering

TL;DR

The paper investigates twist-3 single-spin asymmetries in SIDIS using a scalar diquark spectator model. It demonstrates that gauge-invariant one-loop rescattering generates nonzero $A_{LU}$ and $A_{UL}$ through interference with tree-level amplitudes, indicating that T-odd distributions must be included in a consistent twist-3 description. The findings reveal that previous parton-model formulations are incomplete and that a complete factorization must account for twist-3 T-odd terms, a view later supported by Bacchetta et al. in 2004. Overall, the work highlights the richer twist-3 structure and the need to revise standard interpretations of SIDIS SSAs in experimental analyses.

Abstract

The single spin asymmetries for a longitudinally polarized lepton beam or a longitudinally polarized nucleon target in semi-inclusive deep-inelastic scattering are twist-3 observables. We study these asymmetries in a simple diquark spectator model of the nucleon. Analogous to the case of transverse target polarization, non-vanishing asymmetries are generated by gluon exchange between the struck quark and the target system. It is pointed out that the coupling of the virtual photon to the diquark is needed in order to preserve electromagnetic gauge invariance at the twist-3 level. The calculation indicates that previous analyses of these observables are incomplete.

Figures (2)

• Figure 1: Tree-level diagrams of the process in (\ref{['e:process']}). In order to preserve gauge invariance both diagrams have to be considered.
• Figure 2: One-loop diagrams for the imaginary part of the process in (\ref{['e:process']}). In each diagram the possible on-shell intermediate state is indicated by a thin dotted line. The interaction between the quark and the spectator is modelled by the exchange of an Abelian gauge boson.