Precise determination of the spin structure function g(1) of the proton, deuteron and neutron

Precise measurements of the spin structure functions of the proton g1p(x,Q2) and deuteron g1d(x,Q2) are presented over the kinematic range 0.0041≤x≤0.9 and 0.18  GeV2≤Q2≤20  GeV2. The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function g1n is extracted by combining proton and deuteron data. The integrals of g1p,d at Q2=5  GeV2 are evaluated over the measured x range. Neglecting any possible contribution to the g1d integral from the region x≤0.021, a value of 0.330±0.011(theo)±0.025(exp)±0.028(evol) is obtained for the flavor-singlet axial charge a0 in a leading-twist next-to-next-to-leading-order analysis.