Measurement of the $x$- and $Q^2$-Dependence of the Asymmetry $A_1$ on the Nucleon

Details Measurement of the $x$- and $Q^2$-Dependence of the Asymmetry $A_1$ on the Nucleon Measurement of the $x$- and $Q^2$-Dependence of the Asymmetry $A_1$ on the Nucleon

2006-05-22

arxiv.org/abs/nucl-ex/0605028v1

Phys.Lett.B641:11-17,2006

Physics

Nuclear Physics

Nuclear Experiment

7 pages LaTeX, 5 figures

Scientific paper

10.1016/j.physletb.2006.08.011

We report results for the virtual photon asymmetry $A_1$ on the nucleon from new Jefferson Lab measurements. The experiment, which used the CEBAF Large Acceptance Spectrometer and longitudinally polarized proton ($^{15}$NH$_3$) and deuteron ($^{15}$ND$_3$) targets, collected data with a longitudinally polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present paper, we concentrate on our results for $A_1(x,Q^2)$ and the related ratio $g_1/F_1(x,Q^2)$ in the resonance and the deep inelastic regions for our lowest and highest beam energies, covering a range in momentum transfer $Q^2$ from 0.05 to 5.0 GeV$^2$ and in final-state invariant mass $W$ up to about 3 GeV. Our data show detailed structure in the resonance region, which leads to a strong $Q^2$--dependence of $A_1(x,Q^2)$ for $W$ below 2 GeV. At higher $W$, a smooth approach to the scaling limit, established by earlier experiments, can be seen, but $A_1(x,Q^2)$ is not strictly $Q^2$--independent. We add significantly to the world data set at high $x$, up to $x = 0.6$. Our data exceed the SU(6)-symmetric quark model expectation for both the proton and the deuteron while being consistent with a negative $d$-quark polarization up to our highest $x$. This data setshould improve next-to-leading order (NLO) pQCD fits of the parton polarization distributions.

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