The Spin-dependent Structure Function of the Proton g_1^p and a Test of the Bjorken Sum Rule

TL;DR

This paper reports precise measurements of the proton's spin structure function g_1^p using polarized muon scattering, testing the Bjorken sum rule and providing insights into the nucleon's spin composition.

Contribution

It presents new high-precision measurements of g_1^p and g_1^NS, and confirms the Bjorken sum rule with improved accuracy, advancing understanding of nucleon spin structure.

Findings

First moment of Delta_q3 agrees with Bjorken sum rule prediction

Measured g_1^p(x) with doubled precision at low x

Estimated g_A/g_V ratio consistent with theoretical values

Abstract

The inclusive double-spin asymmetry, A_1^p, has been measured at COMPASS in deepinelastic polarised muon scattering off a large polarised NH3 target. The data, collected in the year 2007, cover the range Q2 > 1 (GeV/c)^2, 0.004 < x < 0.7 and improve the statistical precision of g_1^p(x) by a factor of two in the region x < 0.02. The new proton asymmetries are combined with those previously published for the deuteron to extract the non-singlet spin-dependent structure function g_1^NS(x,Q2). The isovector quark density, Delta_q_3(x,Q2), is evaluated from a NLO QCD fit of g_1^NS. The first moment of Delta_q3 is in good agreement with the value predicted by the Bjorken sum rule and corresponds to a ratio of the axial and vector coupling constants g_A/g_V = 1.28+-0.07(stat)+-0.10(syst).