Narrow Results

The matrix element of the isoscalar axial vector current, , between nucleon states is computed using the external field QCD sum rule method. The external field induced correlator, , is calculated from the spectrum of the isoscalar axial vector meson states. Since it is difficult to ascertain, from QCD sum rule for hyperons, the accuracy of validity of flavor SU(3) symmetry in hyperon decays when strange quark mass is taken into account, we rely on the empirical validity of Cabbibo theory to determine the matrix element between nucleon states. Combining with our calculation of and the well-known nucleon β-decay constant allows us to determine occurring in the Bjorken sum rule. The result is in reasonable agreement with experiment. We also discuss the role of the anomaly in maintaining flavor symmetry and validity of OZI rule.

This study is dedicated to the description of the polarized Bjorken sum rule, based on recently derived formulas within the analytic QCD approach. To accommodate the photoproduction limit and incorporate the Gerasimov–Drell–Hearn and Burkhardt-Cottingham sum rules, we develop a new representation for the twist-2 part of the Bjorken sum rule. The derived results were applied for processing of experimental data. We observed a good agreement between the experimental data and the predictions from analytic QCD. In contrast, there is a significant discrepancy between these data and the fitting curves within the standard perturbative approach.

We present the latest COMPASS results on the proton spin structure function g${}_1^{\mathrm{\text{p}}}$(x) at 200GeV. The data improve the statistical precision by a factor of $\sim$2 at low x. A reevaluation of the Bjorken sum rule based on COMPASS proton and deuteron data confirms its validation to a 9% accuracy. Finally, results from a global NLO QCD fit of g₁ world data are shown. The extracted spin singlet distribution leads to an integrated value of $0.26<\mathrm{\Delta }\mathrm{\Sigma }<0.34$ at Q${}^2=3$ (GeV/c)². The large uncertainty is mainly driven by the unknown shape of the distribution.