Final COMPASS results on the spin-dependent structure functions $g_1^p$ and $g_1^d$ in the deep-inelastic and nonperturbative regions
This paper summarizes the COMPASS Collaboration legacy on
measurements of the proton and deuteron spin-dependent structure functions, g1p and g1d at Q2<1 GeV2 and Q2>1 GeV2. In both regions and at the lowest measured x, g1d(x) is consistent with zero while g1p(x) is positive. This is the first time that the spin effects are observed at such low values of x. The NLO QCD fit of g1 world data gives well constrained quark helicity distributions; gluons are poorly determined. Quark helicity contribution to nucleon spin is 0.26 < DeltaSigma < 0.36. From the COMPASS data alone the Bjorken sum rule is verified to 9% accuracy and the extracted flavour-singlet axial charge is a_0(Q^2 = 3 GeV2)=0.32 +-0.02 (stat.) +-0.04 (syst) +- 0.05 (evol).