Precision measurements of A1n in the deep inelastic regime

We have performed precision measurements of the double-spin virtual-photon asymmetry A1 on the neutron in the deep inelastic scattering regime, using an open-geometry, large-acceptance spectrometer and a longitudinally and transversely polarized 3 He target. Our data cover a wide kinematic range 0.277≤x≤0.548 at an average Q2 value of 3.078 (GeV/c) 2 , doubling the available high-precision neutron data in this x range. We have combined our results with world data on proton targets to make a leading-order extraction of the ratio of polarized-to-unpolarized parton distribution functions for up quarks and for down quarks in the same kinematic range. Our data are consistent with a previous observation of an A1n zero crossing near x=0.5 . We find no evidence of a transition to a positive slope in (Δd+Δd¯)/(d+d¯) up to x=0.548