Spin Structure of the Proton from Polarized Inclusive Deep-Inelastic Muon-Proton Scattering

TL;DR

This paper measures the spin-dependent structure function of the proton using polarized deep-inelastic muon-proton scattering, revealing insights into the proton's spin structure and testing fundamental sum rules in QCD.

Contribution

It provides the first measurement of the proton's $g_1^p$ structure function over a broad kinematic range and performs a QCD analysis to extract the proton's spin contributions.

Findings

The first moment of $g_1^p$ is below the Ellis-Jaffe sum rule prediction.

The singlet axial charge $a_0$ is approximately 0.28.

The data supports the validity of the Bjorken sum rule.

Abstract

We have measured the spin-dependent structure function $g_1^p$ in inclusive deep-inelastic scattering of polarized muons off polarized protons, in the kinematic range $0.003 < x < 0.7$ and $1 GeV^2 < Q^2 < 60 GeV^2$. A next-to-leading order QCD analysis is used to evolve the measured $g_1^p(x,Q^2)$ to a fixed $Q^2_0$. The first moment of $g_1^p$ at $Q^2_0 = 10 GeV^2$ is $\Gamma^p = 0.136\pm 0.013(stat.) \pm 0.009(syst.)\pm 0.005(evol.)$. This result is below the prediction of the Ellis-Jaffe sum rule by more than two standard deviations. The singlet axial charge $a_0$ is found to be $0.28 \pm 0.16$. In the Adler-Bardeen factorization scheme, $\Delta g \simeq 2$ is required to bring $\Delta \Sigma$ in agreement with the Quark-Parton Model. A combined analysis of all available proton and deuteron data confirms the Bjorken sum rule.