Measurement of the x- and Q(2)-dependence of the asymmetry A(1) on the nucleon

Abstract

We report results for the virtual photon asymmetry A I on the nucleon from new Jefferson Lab measurements. The experiment, which used the CEBAF Large Acceptance Spectrometer and longitudinally polarized proton ((NH3)-N-15) and deuteron ((ND3)-N-15) 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 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 J west and highest beam energies, covering a range in momentum transfer Q(2) 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 re-ion, which leads to a strong Q(2) dependence of A(1) (x, Q(2)) to. W below 2 GeV. At higher W, a smooth approach to the scaling limit, established by earlier experiments, can be seen, but A I (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 set should improve next-to-leading order (NLO) pQCD fits of the parton polarization distributions. (c) 2006 Elsevier B.V.All rights reserved.