Spin Physics and Transverse Structure

TL;DR

This paper reviews the role of transverse momentum dependent parton distribution functions (TMDs) in understanding spin and transverse structure in QCD, highlighting their operator complexity and implications for spin physics.

Contribution

It provides an overview of TMD PDFs, emphasizing their operator structure, spin-momentum correlations, and the challenges in universality within QCD.

Findings

TMD PDFs include spin-momentum correlations and T-odd effects.

TMDs extend collinear PDFs to include transverse momentum information.

Operator structure of TMDs is complex, affecting universality.

Abstract

Spin is a welcome complication in the study of partonic structure that has led to new insights, even if theoretically and experimentally not all dust has settled, in particular on quark flavor dependence and gluon spin. At the same time it opened new questions on angular momentum and effects of transverse structure. In this talk the focus is on the role of the transverse momenta of partons. Like for collinear parton distribution functions (PDFs), we are also in the case of transverse momentum dependent (TMD) PDFs, talking about forward matrix elements. TMD PDFs (or in short TMDs) extend collinear PDFs with only spin-spin correlations to PDFs that include spin-momentum correlations, including also time-reversal-odd (T-odd) correlations, relevant for the description of single spin asymmetries. In this way TMDs open up new ways of studying the spin structure. Their operator structure within QCD, however, is more complex leading to various ways of breaking of universality.