Topological charge screening and the ‘proton spin’ beyond the chiral limit

Abstract

The theory of the ‘proton spin’ effect proposed in our earlier papers is extended to include the chiral SU(3) symmetry breaking and flavour mixing induced by non-vanishing quark masses in QCD. The theoretical basis is the derivation of exact, unified Goldberger-Treiman (GT) relations valid beyond the chiral limit. The observed suppression in the flavour singlet axial charge a 0( Q 2) is explained by an anomalously small value for the slope of the singlet current correlation function 〈0| T ϖ μ J μ 5 0 ϖ ν J ν 5 0|0〉, a consequence of the screening of topological charge i QCD vacuum. Numerical predictions are obtained by evaluating the current correlation functions using QCD spectral sum rules. The results, a 0( Q 2) = 0.31 ± 0.02 and ∫ dx g 1 p ( x, Q 2) = 0.141 ± 0.005 (at Q 2 = 10 GeV 2), are in good agreement with current experimental data on the polarised proton structure function g 1 p .