Precision Measurement of the Neutron Spin AsymmetryA1nand Spin-Flavor Decomposition in the Valence Quark Region

We have measured the neutron spin asymmetry A1n with high precision at three kinematics in the deep inelastic region at x=0.33, 0.47, and 0.60, and Q2=2.7, 3.5, and 4.8 (GeV/c)2, respectively. Our results unambiguously show, for the first time, that A1n crosses zero around x=0.47 and becomes significantly positive at x=0.60. Combined with the world proton data, polarized quark distributions were extracted. Our results, in general, agree with relativistic constituent quark models and with perturbative quantum chromodynamics (PQCD) analyses based on the earlier data. However they deviate from PQCD predictions based on hadron helicity conservation.