Spin microscopy with enhanced Wilson lines in the TMD parton densities

TL;DR

This paper introduces enhanced Wilson lines with a Pauli term to directly access spin information in TMDs, preserving leading-twist properties while modifying twist-three anomalous dimensions.

Contribution

It proposes a novel gauge-invariant method using enhanced Wilson lines with a Pauli term to study spin effects in TMDs, extending the theoretical framework.

Findings

Enhanced Wilson lines preserve leading-twist TMD properties.

Modified twist-three anomalous dimensions due to the Pauli term.

Discussion of recent criticisms and theoretical implications.

Abstract

We discuss the possibility of non-minimal gauge invariance of transverse-momentum-dependent parton densities (TMDs) that allows direct access to the spin degrees of freedom of fermion fields entering the operator definition of (quark) TMDs. This is achieved via enhanced Wilson lines that are supplied with the spin-dependent Pauli term $\sim F^{\mu\nu}[\gamma_\mu, \gamma_\nu]$, thus providing an appropriate tool for the "microscopic" investigation of the spin and color structure of TMDs. We show that this generalization leaves the leading-twist TMD properties unchanged but modifies those of twist three by contributing to their anomalous dimensions. We also comment on Collins' recent criticism of our approach.