Single-spin asymmetries at two loops

TL;DR

This paper introduces a new mechanism for generating transverse single-spin asymmetry in semi-inclusive deep inelastic scattering, arising from two-loop parton-level processes, expanding understanding beyond known effects like Sivers and Collins functions.

Contribution

It identifies a novel two-loop mechanism for SSA, detailing the complete set of diagrams and their gauge invariance, and discusses implications for collinear and TMD factorization frameworks.

Findings

A phase for SSA can be generated purely within two-loop parton processes.

Complete set of two-loop diagrams for SSA identified and analyzed.

Contributions from all two-parton TMD PDFs up to two loops and twist three are presented.

Abstract

We find a novel mechanism for generating transverse single-spin asymmetry (SSA) in semi-inclusive deep inelastic scattering, distinct from the known ones which involve the Sivers and Collins functions, or their collinear twist-three counterparts. It is demonstrated that a phase needed for SSA can be produced purely within a parton-level cross section starting at two loops. We identify the complete set of two-loop diagrams for SSA, and discuss their gauge invariance and collinear factorization which features the $g_T$ distribution function. In the $k_T$ factorization framework, many more sources for SSA exist, and contributions from all possible two-parton transverse-momentum-dependent parton distribution functions are presented up to two loops and twist three.