The spin structure function g 1 p of the proton and a test of the Bjorken sum rule

摘要

New results for the double spin asymmetry A 1 p and the proton longitudinal spin structure function g 1 p are presented. They were obtained by the COMPASS Collaboration using polarised 200 GeV muons scattered off a longitudinally polarised NH 3 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 g 1 p ( x ) by about a factor of two in the region x ? 0.02 . A next-to-leading order QCD fit to the g 1 world data is performed. It leads to a new determination of the quark spin contribution to the nucleon spin, ΔΣ, ranging from 0.26 to 0.36, and to a re-evaluation of the first moment of g 1 p . The uncertainty of ΔΣ 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 | g A / g V | = 1.22 ± 0.05 ? ( stat. ) ± 0.10 ? ( syst. ) , which validates the sum rule to an accuracy of about 9%.