Measurement of the x- and Q2-dependence of the asymmetry A1 on the nucleon
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Abstract
We report results for the virtual photon asymmetry A1 on the nucleon from new Jefferson Lab measurements. The experiment, which used the CEBAF Large Acceptance Spectrometer and longitudinally polarized proton (NH315) and deuteron (ND315) targets, collected data with a longitudinally polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present Letter, we concentrate on our results for A1(x,Q2) and the related ratio g1/F1(x,Q2) in the resonance and the deep inelastic regions for our lowest and highest beam energies, covering a range in momentum transfer Q2 from 0.05 to 5.0(GeV/c)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 Q2-dependence of A1(x,Q2) for W below 2 GeV. At higher W, a smooth approach to the scaling limit, established by earlier experiments, can be seen, but A1(x,Q2) is not strictly Q2-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 set should improve next-to-leading order (NLO) pQCD fits of the parton polarization distributions.
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