Precision measurements of A^n_1 in the deep inelastic regime

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 ^3He 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 (Δd+Δd)/(d+d) up to x=0.548x=0.548.