Measurement of the Proton Spin Structure Function $g_1(x,Q^2)$ for $Q^2$ from 0.15 to 1.6 GeV${}^2$ with CLAS

Abstract

Double-polarization asymmetries for inclusive $ep$ scattering were measured at Jefferson Lab using 2.6 and 4.3 GeV longitudinally polarized electrons incident on a longitudinally polarized NH${}_3$ target in the CLAS detector. The polarized structure function $g_1(x,Q^2)$ was extracted throughout the nucleon resonance region and into the deep inelastic regime, for $Q^2=0.15-1.64$GeV${}^2$. The contributions to the first moment $\hat{I}{“}_1(Q^2)=\int g_1(x,Q^2)dx$ were determined up to $Q^2=1.2$ GeV${}^2$. Using a parametrization for $g_1$ in the unmeasured low $x$ regions, the complete first moment was estimated over this $Q^2$ region. A rapid change in $\hat{I}{“}_1$ is observed for $Q^2<1$GeV${}^2$, with a sign change near $Q^2=0.3$GeV${}^2$, indicating dominant contributions from the resonance region. At $Q^2=1.2$ GeV${}^2$ our data are below the pQCD evolved scaling value.

6 pages, 3 figures, submitted to Phys. Rev. Lett