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The Spin Structure of the Nucleon

B. W. Filippone, Xiangdong Ji

TL;DR

The paper surveys the spin structure of the nucleon, framing the problem in QCD and detailing how spin contributions arise from quarks and gluons, including their orbital angular momenta. It surveys experimental progress across SLAC, CERN, DESY, and RHIC, and outlines methods to extract the total quark helicity ΔΣ, individual flavor helicities, and the gluon helicity ΔG through inclusive and semi-inclusive DIS, jet and charm production, direct photons, and polarized hadron collisions, with emphasis on NLO QCD evolution and factorization schemes. It also discusses transverse-spin phenomena via g2 and transversity, and introduces off-forward parton distributions and deeply virtual Compton scattering as probes of angular-momentum structure, along with related topics such as the DHG sum rule generalizations and Λ fragmentation. Overall, the work highlights how diverse experimental approaches and advanced QCD analyses converge to map the spin decomposition of the nucleon and points to future facilities (e.g., RHIC, polarized HERA, DVCS measurements) as key to resolving remaining uncertainties in ΔG and orbital components.

Abstract

We present an overview of recent experimental and theoretical advances in our understanding of the spin structure of protons and neutrons.

The Spin Structure of the Nucleon

TL;DR

The paper surveys the spin structure of the nucleon, framing the problem in QCD and detailing how spin contributions arise from quarks and gluons, including their orbital angular momenta. It surveys experimental progress across SLAC, CERN, DESY, and RHIC, and outlines methods to extract the total quark helicity ΔΣ, individual flavor helicities, and the gluon helicity ΔG through inclusive and semi-inclusive DIS, jet and charm production, direct photons, and polarized hadron collisions, with emphasis on NLO QCD evolution and factorization schemes. It also discusses transverse-spin phenomena via g2 and transversity, and introduces off-forward parton distributions and deeply virtual Compton scattering as probes of angular-momentum structure, along with related topics such as the DHG sum rule generalizations and Λ fragmentation. Overall, the work highlights how diverse experimental approaches and advanced QCD analyses converge to map the spin decomposition of the nucleon and points to future facilities (e.g., RHIC, polarized HERA, DVCS measurements) as key to resolving remaining uncertainties in ΔG and orbital components.

Abstract

We present an overview of recent experimental and theoretical advances in our understanding of the spin structure of protons and neutrons.

Paper Structure

This paper contains 46 sections, 137 equations, 23 figures, 6 tables.

Table of Contents

  1. Introduction
  2. A Simple Model for Proton Spin
  3. Lepton Scattering as a Probe of Spin Structure
  4. Theoretical Introduction
  5. Quark Helicity Distributions and $g_1(x, Q^2)$
  6. The Nucleon Spin Sum Rule
  7. Gluon Helicity Distribution $\Delta G(x,Q^2)$
  8. Experimental Overview
  9. SLAC Experiments
  10. CERN Experiments
  11. DESY Experiments
  12. RHIC Spin Program
  13. Total Quark Helicity Distribution
  14. Virtual Photon Asymmetries
  15. Extraction of $g_1(x,Q^2)$
  16. ...and 31 more sections

Figures (23)

  • Figure 1: Deep Inelastic Scattering in the Quark-Parton Model
  • Figure 2: Schematic diagram of SLAC E142/E143 spectrometers
  • Figure 3: Schematic diagram of SMC spectrometer
  • Figure 4: Schematic diagram of HERMES spectrometer
  • Figure 5: Schematic diagram of the STAR detector
  • ...and 18 more figures