Twist-3 in Proton Nucleon Single Spin Asymmetries

TL;DR

This work analyzes twist-3 contributions to proton-nucleon single spin asymmetries, arguing that quark-gluon correlations generate sizable effects without strong suppression by quark masses, and that a QCD-factorization framework with two-argument correlators describes the long-distance physics. It derives Wandzura–Wilczek-type relations for $g_2$, discusses potential BC-sum-rule violations, and introduces a simple model for the twist-3 part of $g_2$ consistent with sum rules and Regge behavior. The paper compares gluonic-pole and fermionic-pole contributions, finding the latter potentially dominant, and provides quantitative predictions for $ar{g}_2(x)$ with specific proton and neutron values near $x o0.1$. It also outlines short-distance subprocess calculations and advocates jet- and dilepton-left-right asymmetries as robust experimental probes of twist-3 QCD effects, including proposed measurements at high-energy facilities. Overall, the results guide the interpretation of single-spin phenomena and motivate nonperturbative studies (sum rules, lattice) to quantify the fermionic versus gluonic pole contributions.

Abstract

These proceedings contain the 'physical' derivation of the Wandzura-Wilczek relations for the spin-dependent structure function g2 (amounting to keeping the intrinsic transverse momentum of quarks, and neglecting the physical transverse components of the gluon fields), recently generalized for the case of non-forward processes (hep-ph/0102209). The simple numerical model for the twist-3 contribution to g2 was also suggested.