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, Δg2 is required to bring {Δ}{Σ} in agreement with the quark-parton model. A combined analysis of all available proton, deuteron, and ³He data confirms the Bjorken sum rule.