[Paper Review] Spin Physics: Session Summary

C. Aidala, Simonetta Liuti+1 more|arXiv (Cornell University)|Jul 26, 2010

Quantum Chromodynamics and Particle Interactions16 references3 citations

TL;DR

This paper summarizes key advances in spin physics from the DIS 2010 conference, focusing on the extraction and theoretical interpretation of Generalized Parton Distributions (GPDs) and Transverse Momentum Distributions (TMDs) from deep inelastic scattering and exclusive processes. It highlights improved constraints on the gluon spin contribution ($\Delta G \approx 0$) and evidence for non-zero orbital angular momentum in valence quarks, with GPDs providing access to the nucleon's 3D partonic structure via interference effects in $\omega$ and $\phi$ meson production.

ABSTRACT

We summarize the main results of the Spin Physics Working Group Sessions at DIS 2010, the XVIII International Workshop on Deep Inelastic Scattering and Related Subjects.

Motivation & Objective

• To assess the state-of-the-art in spin physics, particularly the measurement and theoretical interpretation of GPDs and TMDs in the context of the LHC era.
• To improve constraints on the nucleon's spin structure, especially the gluon polarization ($\Delta G$) and quark orbital angular momentum (OAM).
• To evaluate the role of GPDs and TMDs in providing a 3D 'holographic' picture of the nucleon's partonic structure.
• To examine the feasibility of global fits for GPDs and the challenges in their partonic interpretation, especially in the ERBL region.
• To explore the applicability of GPD and TMD formalisms to future LHC physics, including multi-parton interactions and SUSY production.

Proposed method

• Analysis of experimental data from HERMES, Jefferson Lab, CERN/Compass, BNL, and KEK on semi-inclusive deep inelastic scattering (SIDIS) and exclusive processes.
• Use of helicity and transverse-spin asymmetries in exclusive meson production (e.g., $\omega$, $\phi$) to probe interference between GPDs $H^q$ and $E^q$.
• Application of dispersion relations to relate CFFs (Compton form factors) to GPDs, reducing kinematical variables and addressing $t$-channel thresholds.
• Employment of light-front wave functions and model-based approaches to interpret GPDs in the ERBL region, where quark momenta are negative.
• Development of formalisms for factorization and evolution of unintegrated parton distributions, including error propagation in parametrizations.
• Use of lattice QCD calculations to validate Ji's sum rule and probe total angular momentum contributions.

Experimental results

Research questions

• RQ1 What is the current experimental constraint on the gluon spin contribution ($\Delta G$) to the nucleon spin?
• RQ2 How can transverse target-spin asymmetries in exclusive meson production constrain the $E^q$ GPDs and reveal information about quark orbital angular momentum?
• RQ3 What is the partonic interpretation of GPDs in the ERBL region, and how do multi-parton components and reinteractions affect this interpretation?
• RQ4 To what extent can dispersion relations and kinematical reductions simplify the extraction of GPDs from CFFs in exclusive processes?
• RQ5 Can GPD-based formalisms be applied to model multi-parton interactions and polarization effects in LHC processes such as SUSY hadroproduction?

Key findings

• Recent RHIC measurements indicate that the gluon spin contribution ($\Delta G$) to the nucleon spin is very small or compatible with zero.
• Preliminary HERMES data on single-spin asymmetries in exclusive $\omega$ and $\phi$ meson production show a negative asymmetry for $\omega$-mesons, consistent with model predictions and sensitive to the interference between $H^q$ and $E^q$ GPDs.
• The asymmetry for $\phi$-mesons (pure $s\bar{s}$) is compatible with zero, suggesting that the $E$ GPDs for sea quarks and gluons may be negligible.
• The sign of the transverse-spin asymmetry in $\omega$ production provides evidence for non-vanishing orbital angular momentum of valence quarks, with a measurable sign dependence.
• Theoretical analysis suggests that all GPD information may be concentrated on the kinematical ridge between the ERBL and DGLAP regions, implying a potential simplification in the formalism.
• Lattice QCD calculations have reached a stage where Ji's sum rule can be validated, enabling a direct link between GPDs and total angular momentum.