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

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 ${\mathrm{N}\mathrm{H}}_3$ 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 $Q^2=0.15–1.64{\mathrm{G}\mathrm{e}\mathrm{V}}^2$. The contributions to the first moment ${\Gamma }_1(Q^2)=\int g_1(x,Q^2)dx$ were determined up to $Q^2=1.2{\mathrm{G}\mathrm{e}\mathrm{V}}^2$. 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 ${\Gamma }_1$ is observed for $Q^2<1{\mathrm{G}\mathrm{e}\mathrm{V}}^2$, with a sign change near $Q^2=0.3{\mathrm{G}\mathrm{e}\mathrm{V}}^2$, indicating dominant contributions from the resonance region. At $Q^2=1.2{\mathrm{G}\mathrm{e}\mathrm{V}}^2$ our data are below the perturbative QCD evolved scaling value.