The spin-dependent structure function of the proton g1p and a test of the Bjorken sum rule

The inclusive double-spin asymmetry. A(1)(p), has been measured at COMPASS in deep-inelastic polarised muon scattering off a large polarised NH3 target. The data, collected in the year 2007, cover the range Q(2) > 1 (GeV/c)(2), 0.004 < x < 0.7 and improve the statistical precision of g(1)(p)(x) by a factor of two in the region x < 0.02. The new proton asymmetries are combined with those previously published for the deuteron to extract the non-singlet spin-dependent structure function g(1)(NS)(x. Q(2)). The isovector quark density, Delta q(3)(x, Q(2)), is evaluated from a NLO QCD fit of g(1)(NS). The first moment of Delta q(3) is in good agreement with the value predicted by the Bjorken sum rule and corresponds to a ratio of the axial and vector coupling constants vertical bar g(A)/g(V)vertical bar = 1.28 +/- 0.07 (stat.) +/- 0.10 (syst.). (C) 2010 Elsevier B.V. All rights reserved.