Evidence for Quark-Hadron Duality in the Proton Spin Asymmetry $A_1$

Spin-dependent lepton-nucleon scattering data have been used to investigate the validity of the concept of quark-hadron duality for the spin asymmetry $A_1$. Longitudinally polarized positrons were scattered off a longitudinally polarized hydrogen target for values of $Q^2$ between 1.2 and $12{\mathrm{G}\mathrm{e}\mathrm{V}}^2$ and values of $W^2$ between 1 and $4{\mathrm{G}\mathrm{e}\mathrm{V}}^2$. The average double-spin asymmetry in the nucleon resonance region is found to agree with that measured in deep-inelastic scattering at the same values of the Bjorken scaling variable $x$. This finding implies that the description of $A_1$ in terms of quark degrees of freedom is valid also in the nucleon resonance region for values of $Q^2$ above $1.6{\mathrm{G}\mathrm{e}\mathrm{V}}^2$.

Figure 1

Spin asymmetry $A_1$ as a function of $x$ measured in the resonance region (full circles). Error bars represent the statistical uncertainties; the systematic uncertainty for the data in the resonance region is about 16%. Open symbols are previous results obtained in the DIS region. The curve represents a power law fit to DIS data at $x>0.3$.