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

Abstract

We have measured the spin-dependent structure function g(1)(p) in inclusive deep-inelastic scattering of polarized muons off polarized protons, in the kinematic range 0.003<x<0.7 and I GeV2<Q(2)<60 GeV2. A next-to-leading order QCD analysis is used to evolve the measured g(1)(p)(x,Q(2)) to a fixed Q(0)(2). The first moment of g(1)(p) at Q(0)(2)=10 GeV2 is Gamma=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 a(0) is found to be 0.28+/-0.16. Ln the Adler-Bardeen factorization scheme, Delta g similar or equal to 2 is required to bring Delta Sigma in agreement with the quark-parton model. A combined analysis of all available proton, deuteron, and He-3 data confirms the Bjorken sum rule.