Theoretical aspects of spin physics

TL;DR

The paper surveys how spin phenomena are described within collinear factorization and extends to transverse-momentum dependent frameworks, emphasizing the gauge-invariant definitions of TMDs like the Sivers and Collins functions. It discusses the theoretical interplay between Sivers/Qiu-Sterman effects, process dependence due to Wilson lines, and the role of Sudakov factors in scale evolution of azimuthal single-spin asymmetries. A key message is that large SSAs demand beyond-collinear approaches, with a tight link between twist-3 and TMD formalisms and a need to understand Q^2 evolution and process-dependent signs between SIDIS and Drell-Yan. The work highlights ongoing theoretical development and experimental implications for accessing transversity and spin-orbit correlations in hadrons.

Abstract

A summary is given of how spin enters in collinearly factorizing processes. Next, theoretical aspects of polarization in processes beyond collinear factorization are discussed in more detail, with special focus on recent developments concerning the color gauge invariant definitions of transverse momentum dependent distribution and fragmentation functions, such as the Sivers and Collins effect functions. This has particular relevance for azimuthal single spin asymmetries, which currently receive much theoretical and experimental attention.

Figures (10)

• Figure 1: The $\gamma^* p$ cross section can be expanded in terms of parton correlators.
• Figure 2: The two-quark correlation function $\Phi$, which depends on the hadron momentum ${P}$, quark momentum ${p=xP}$ and hadron spin vector ${S}$.
• Figure 3: The helicity dependence of transversity $\delta q$ and the twist-3 function $g_T$.
• Figure 4: Pictorial representations of the Qiu-Sterman effect, the Sivers effect and the contribution considered by Brodsky, Hwang and Schmidt to generate nonzero SSA.
• Figure 5: The links in DIS (l.h.s.) and DY run in opposite directions along the lightcone towards lightcone infinity, where an excursion in the transverse direction is taken.