Spin structure of the proton from polarized inclusive deep-inelastic muon-proton scattering

We have measured the spin-dependent structure function g1p in inclusive deep-inelastic scattering of polarized muons off polarized protons, in the kinematic range 0.003<x<0.7 and 1GeV2<Q2<60GeV2. A next-to-leading order QCD analysis is used to evolve the measured g1p(x,Q2) to a fixed Q02. The first moment of g1p at Q02=10GeV2 is Γ1p=0.136±0.013 (stat) ±0.009 (syst) ±0.005 (evol). This result is below the prediction of the Ellis-Jaffe sum rule by more than two standard deviations. The singlet axial charge a0 is found to be 0.28±0.16. In the Adler-Bardeen factorization scheme, Δg≃2 is required to bring ΔΣ in agreement with the quark-parton model. A combined analysis of all available proton, deuteron, and 3He data confirms the Bjorken sum rule.