Experimental study of isovector spin sum rules

TL;DR

This paper analyzes the Bjorken integral and spin polarizability data from Jefferson Lab to study higher twist effects and test theoretical models, revealing significant findings about twist-4 contributions and discrepancies with Chiral Perturbation Theory.

Contribution

It provides the first extraction of the twist-4 element $f_{2}^{p-n}$ from experimental data and performs an isospin decomposition of the generalized forward spin polarizability.

Findings

The twist-4 element $f_{2}^{p-n}$ is large and negative.

The isovector part of $ extgamma_0$ disagrees with Chiral Perturbation Theory calculations.

Systematic studies support the validity of higher twist analysis at $Q^{2} \\sim 1$ GeV$^2$.

Abstract

We present the Bjorken integral extracted from Jefferson Lab experiment EG1b for $0.05<Q^{2}<2.92$ GeV$^2$. The integral is fit to extract the twist-4 element $f_{2}^{p-n}$ which appears to be relatively large and negative. Systematic studies of this higher twist analysis establish its legitimacy at $Q^{2}$ around 1 GeV$^{2}$. We also performed an isospin decomposition of the generalized forward spin polarizability $\gamma_{0}$. Although its isovector part provides a reliable test of the calculation techniques of Chiral Perturbation Theory, our data disagree with the calculations.