Spin Physics and Polarized Structure Functions

TL;DR

This paper surveys the theoretical framework and experimental status of spin physics in polarized deep inelastic scattering and related hard processes, detailing how the polarized structure function $g_1$ encodes quark and gluon helicities via $\Delta\Sigma$ and $\Delta g$ within the QCD-improved parton model. It discusses LO and NLO QCD evolution, the axial anomaly, and operator-product expansion, and explains how Bjorken and DHG sum rules constrain spin sum rules while highlighting scheme dependencies. The authors review early SLAC/CERN data, neutron extractions from ${}^3\mathrm{He}$ and deuteron targets, and modern global analyses that fit polarized parton densities to $g_1^{p,n,d}$ data, including heavy-quark production and semi-inclusive channels to access $\Delta g$ and flavor separation. They emphasize the spin decomposition of the nucleon, the role of higher-twist effects at low $Q^2$, and the experimental prospects at RHIC, HERMES, and future polarized facilities for clarifying the proton’s spin budget and the dynamics of spin in QCD.

Abstract

A review on the theoretical aspects and the experimental results of polarized deep inelastic scattering and of other hard scattering processes is presented. The following items are discussed: longitudinally polarized structure functions, results from the SLAC and CERN polarization experiments, the QCD interpretation and the LO and NLO Q2-evolution of g1, the role of the polarized gluon density, the expectations for x-->0, sum rules, the first moment of the polarized structure function, the parametrizations of polarized parton densities, polarized jet, heavy quark and direct photon production, DIS semi-inclusive asymmetries and elastic neutrino-proton scattering, single and double spin asymmetries, structure functions for higher spin hadrons and nuclei, nonperturbative approaches, the transverse structure function g2, chiral-odd 'transversity' distributions.

Figures (55)

• Figure 1: The basic polarized deep inelastic scattering process
• Figure 2: The definition of the lepton tensor
• Figure 3: The basic form of the polarized experiments
• Figure 4: The geometry of the polarized deep inelastic scattering process in the lab frame
• Figure 5: The spin asymmetry of the proton from the "old" SLAC data from 1976 and 1983 hughes1. In the naive SU(6) model one has $A_1^p={5 \over 9}$ and $A_1^n=0$close.