Proton Spin Structure in the Resonance Region

We have examined the spin structure of the proton in the region of the nucleon resonances ($1.085\mathrm{GeV}<W<1.910\mathrm{GeV}$) at an average four momentum transfer of $Q^2=1.3{\mathrm{GeV}}^2$. Using the Jefferson Lab polarized electron beam, a spectrometer, and a polarized solid target, we measured the asymmetries $A_{\parallel }$ and $A_{\perp }$ to high precision, and extracted the asymmetries $A_1$ and $A_2$, and the spin structure functions $g_1$ and $g_2$. We found a notably nonzero $A_{\perp }$, significant contributions from higher-twist effects, and only weak support for polarized quark-hadron duality.

Figure 1

Our measured asymmetries $A_{\parallel }$ and $A_{\perp }$, fully corrected (points) and without radiative corrections (curves).

Figure 2

Virtual photon asymmetries $A_1$ and $A_2$ from our data and corresponding fits. Also shown is the E155 fit to DIS data [25, 26], evaluated at our ($x$, $Q^2$), and the Soffer limit for $A_2$ [31], based on our $A_1$ fit. The upper error band indicates the systematic error in $A_1$, the lower one $A_2$.

Figure 3

Results for $g_1$ from this experiment (RSS) and other relevant data [2, 32], as well as target mass corrected NLO PDFs. The upper scale shows $W$ (at $Q^2=1.3{\mathrm{GeV}}^2$) for reference.

Figure 4

Our (RSS) values for $g_2$ and the approximation $g_2^{\mathrm{WW}}$ [Eq. (1)] as evaluated from our data.