We have performed precision measurements of the double-spin virtual-photon asymmetry A 1 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 Q 2 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 A 1 n zero crossing near x = 0.5 . We find no evidence of a transition to a positive slope in ( Δ d + Δ d ¯ ) / ( d + d ¯ ) up to x = 0.548 .
