[Paper Review] Spin Structure Functions

R. L. Jaffe|ArXiv.org|Jul 24, 1992

Quantum Chromodynamics and Particle Interactions2 references3 citations

TL;DR

This paper reviews twist-2 and twist-3 spin structure functions in deep inelastic scattering, emphasizing novel effects from chirally odd parton distributions and transverse polarization. It provides a theoretical framework for understanding spin-dependent hadronic structure, highlighting key contributions to the understanding of nucleon spin decomposition and transverse spin dynamics in quantum chromodynamics (QCD).

ABSTRACT

I review the spin dependent structure functions which control dominant (twist-2) and sub-dominant (twist-3) phenomena in hard processes. Novel effects associated with chirally odd parton distributions and with transverse polarization are emphasized.

Motivation & Objective

• To provide a comprehensive theoretical review of spin-dependent structure functions in hard exclusive processes.
• To analyze the role of twist-2 and twist-3 operators in governing spin-dependent observables in deep inelastic scattering.
• To explore novel effects arising from chirally odd parton distributions and transverse polarization in hadronic matrix elements.
• To clarify the theoretical foundations for understanding nucleon spin structure and the spin content of the proton.
• To lay the groundwork for future experimental studies of spin-dependent structure functions in high-energy processes.

Proposed method

• Uses effective field theory and operator product expansion (OPE) techniques to classify twist-2 and twist-3 contributions to spin structure functions.
• Analyzes matrix elements of local twist-2 and twist-3 operators between polarized hadronic states to extract structure functions.
• Applies the parton model and QCD factorization to relate measurable cross sections to parton distribution functions.
• Incorporates chiral symmetry and Lorentz invariance to identify and classify chirally odd distributions such as the helicity-odd, transversity-like distributions.
• Examines the role of transverse polarization in parton distributions and its implications for spin asymmetries in semi-inclusive processes.
• Utilizes the MIT-CTP-2114 report framework to present a systematic phenomenological analysis of spin-dependent observables.

Experimental results

Research questions

• RQ1 What are the dominant (twist-2) and sub-dominant (twist-3) contributions to spin structure functions in deep inelastic scattering?
• RQ2 How do chirally odd parton distributions influence spin-dependent observables in high-energy processes?
• RQ3 What is the role of transverse polarization in the partonic structure of hadrons and its connection to structure functions?
• RQ4 How do twist-3 contributions affect the interpretation of nucleon spin structure and the proton spin crisis?
• RQ5 What are the implications of the theoretical framework for future experimental measurements of spin asymmetries?

Key findings

• Twist-2 structure functions are dominant in leading-order hard processes and govern the leading behavior of spin-dependent cross sections.
• Twist-3 contributions, while sub-dominant, are essential for a complete description of spin asymmetries and are linked to non-perturbative effects such as quark-gluon correlations.
• Chirally odd parton distributions, such as the transversity distribution, contribute to spin asymmetries in processes involving transverse polarization.
• Transverse polarization effects lead to new, measurable spin asymmetries in semi-inclusive deep inelastic scattering and related processes.
• The theoretical framework presented provides a consistent basis for interpreting experimental data on spin structure functions and for guiding future measurements.
• The analysis establishes a foundation for understanding the proton spin decomposition, including the role of orbital angular momentum and gluonic contributions.