Precision Measurements of A\u3csup\u3en\u3c/sup\u3e\u3csub\u3e1\u3c/sub\u3e 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 3He 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 An1 zero crossing near x = 0.5. We find no evidence of a transition to a positive slope in ( d + d¯)/(d + d¯) up to x = 0.548