The Spin Structure Function $g_1^{\rm p}$ of the Proton and a Test of the Bjorken Sum Rule
New results for the double spin asymmetry $A_1^{\rm p}$ and the protonlongitudinal spin structure function $g_1^{\rm p}$ are presented. They wereobtained by the COMPASS collaboration using polarised 200 GeV muons scatteredoff a longitudinally polarised NH$_3$ target. The data were collected in 2011and complement those recorded in 2007 at 160\,GeV, in particular at lowervalues of $x$. They improve the statistical precision of $g_1^{\rm p}(x)$ byabout a factor of two in the region $x\lesssim 0.02$. A next-to-leading orderQCD fit to the $g_1$ world data is performed. It leads to a new determinationof the quark spin contribution to the nucleon spin, $\Delta \Sigma$ rangingfrom 0.26 to 0.36, and to a re-evaluation of the first moment of $g_1^{\rm p}$.The uncertainty of $\Delta \Sigma$ is mostly due to the large uncertainty inthe present determinations of the gluon helicity distribution. A new evaluationof the Bjorken sum rule based on the COMPASS results for the non-singletstructure function $g_1^{\rm NS}(x,Q^2)$ yields as ratio of the axial andvector coupling constants $|g_{\rm A}/g_{\rm V}| = 1.22 \pm 0.05~({\rm stat.})\pm 0.10~({\rm syst.})$, which validates the sum rule to an accuracy of about9\%.
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18-28
18-28
Elsevier Science BV