Low Q2 Measurement of g2p and the LT Spin Polarizability

TL;DR

This paper discusses low Q2 measurements of the proton g2 structure function at JLab, aiming to test sum rules, validate theoretical models, and improve understanding of proton spin structure in the resonance and transition regions.

Contribution

It presents new experimental data on g2^p at low Q2, providing critical benchmarks for Chiral Perturbation Theory and insights into proton hyperfine structure.

Findings

Data will test the Burkhardt-Cottingham sum rule.

Results will help extract higher moments like δ_LT^p and d_2^p.

Measurements will clarify the transition from resonance to parton behavior.

Abstract

JLab has been at the forefront of a program to measure the nucleon spin-dependent structure functions over a wide kinematic range, and data of unprecedented quality has been extracted in all three experimental halls. Moments of these quantities have proven to be powerful tools to test QCD sum rules and provide benchmark tests of Lattice QCD and Chiral Perturbation Theory. Precision measurements of $g_{1,2}^n$ and $g_1^p$ have been performed as part of the highly successful `extended GDH program', but data on the $g_2^p$ structure function remain scarce. We discuss here JLab experiment E08-027, which will measure quantity $g_2^p$ in the resonance region at low $Q^2$. These data will be used to test the Burkhardt-Cottingham sum rule and to extract the higher moments $\delta_{LT}^p(Q^2)$ and $\overline{d}_2^p(Q^2)$. Data in the $Q^2$ range $0.02<Q^2<0.4$ GeV$^2$ will provide unambiguous benchmark tests of $\chi$PT calculations on the lower end, while probing the transition region at the high $Q^2$ end where parton-like behaviour begins to emerge. This data will also have a significant impact on our theoretical understanding of the hyperfine structure of the proton, and reduce the systematic uncertainty of previous experiments which extracted the $g_1^p$ structure from purely longitudinal measurements.