The proton spin puzzle: where are we today ?

Abstract

The proton spin puzzle has challenged our understanding of QCD for the last 20 years. New measurements of polarized glue, valence and sea quark polarization, including strange quark polarization, are available. What is new and exciting in the data, and what might this tell us about the structure of the proton ? The proton spin puzzle seems to be telling us about the interplay of valence quarks with the complex vacuum structure of QCD.

Invited Brief Review for Modern Physics Letters A, 17 pages

References in corpus (30)

• Constituent Quarks and g_1
• Models for the Polarized Parton Distributions of the Nucleon
• Precise determination of the spin structure function ${\mathbf{g}}_1$ of the proton, deuteron and neutron
• The Deuteron Spin-dependent Structure Function g1d and its First Moment
• Prospects for Spin Physics at RHIC
• Consistent Analysis of the Spin Content of the Nucleon
• Polarised Quark Distributions in the Nucleon from Semi-Inclusive Spin Asymmetries
• Measurement of the Spin Asymmetry in the Photoproduction of Pairs of High-pT Hadrons at HERMES
• Measurement of Parton Distributions of Strange Quarks in the Nucleon from Charged-Kaon Production in Deep-Inelastic Scattering on the Deuteron
• Gluon polarization in the nucleon from quasi-real photoproduction of high-pT hadron pairs
• Flavor decomposition of the sea quark helicity distributions in the nucleon from semi-inclusive deep-inelastic scattering
• Inclusive cross section and double helicity asymmetry for pi^0 production in p+p collisions at sqrt(s) = 62.4 GeV
• The Polarised Valence Quark Distribution from semi-inclusive DIS
• The gluon spin contribution to the proton spin from the double helicity asymmetry in inclusive pi^0 production in polarized p+p collisions at sqrt(s)=200 GeV
• Impact of CLAS and COMPASS data on Polarized Parton Densities and Higher Twist
• Gluon Spin in the Nucleon
• The (alpha_s)^3 approximation of Quantum Chromodynamics to the Ellis-Jaffe sum rule
• Target Independence of the Emc-SMC Effect
• The Spin Dependence of Diffractive Processes and Implications for the Small $x$ Behaviour of $g_1$ and the Spin Content of the Nucleon
• Perturbative Evolution and Regge Behaviour
• Gluons and the eta' nucleon coupling constant
• Gluon Polarisation in the Nucleon and Longitudinal Double Spin Asymmetries from Open Charm Muoproduction
• The small $x$ behaviour of $g_1$
• Can the polarization of the strange quarks in the proton be positive ?
• Heavy-quark axial charges to non-leading order
• A polarised QCD condensate: nu p elastic scattering as a probe of U_A(1) dynamics
• The Spin Structure of the Proton and Polarized Collider Physics
• Transverse Momentum in Semi-Inclusive Polarized Deep Inelastic Scattering and the Spin-Flavor Structure of the Proton
• Spin constraints on Regge predictions and perturbative evolution in high energy collisions
• Jet Production in Polarized pp Collisions at RHIC

Cited by in corpus (10)

• Theory of Radiative Electron Polarization in Strong Laser Fields
• The nucleon's octet axial-charge g_A^8 with chiral corrections
• Measurement of the parity-violating longitudinal single-spin asymmetry for $W^{\pm }$ boson production in polarized proton-proton collisions at $\sqrt{s}=500$GeV
• Particle physics experiments based on the AWAKE acceleration scheme
• Gluon polarization in the proton
• A still unsettled issue in the nucleon spin decomposition problem : On the role of surface terms and gluon topology
• Effect of electron spin polarization in laser-assisted electron-proton scattering
• Particle Physics in the LHC Era and beyond
• DIS 2009 Concluding Talk: Outlook and Perspective
• A non-Standard Standard Model