The spin structure function gp1 of the proton and a test of the Bjorken sum rule

New results for the double spin asymmetry Ap1 and the proton longitudinal spin structure function g1p are presented. They were obtained by the COMPASS Collaboration using polarised 200 GeV muons scattered off a longitudinally polarised NH3 target. The data were collected in 2011 and complement those recorded in 2007 at 160 GeV, in particular at lower values of x. They improve the statistical precision of g1p(x) by about a factor of two in the region x<0.02. A next-to-leading order QCD fit to the g1 world data is performed. It leads to a new determination of the quark spin contribution to the nucleon spin, Delta Sigma, ranging from 0.26 to 0.36, and to a re-evaluation of the first moment of g1p. The uncertainty of is mostly due to the large uncertainty in the present determinations of the gluon helicity distribution. A new evaluation of the Bjorken sum rule based on the COMPASS results for the non-singlet structure function g_1^NS(x, Q 2) yields as ratio of the axial and vector coupling constants |gA/gV| = 1.22 ±0.05 (stat.) ±0.10 (syst.), which validates the sum rule to an accuracy of about 9%.