In Search of the Quark Spins in the Nucleon: A Next--to--Next--to-- Leading Order QCD Analysis

Abstract

The data from the last seven experiments performed on polarized deep--inelastic scattering on proton and neutron (or deuteron) targets have been analyzed in search of a precise determination of the spin fraction carried by the quarks in the nucleon. We find that this fraction can be of the size expected from na\"ıve quark model arguments, provided the gluon axial anomaly is explicitly included and the isosinglet axial charge normalization is fixed at a suitably low momentum scale, such that a) the running, strong coupling constant is about unit, and b) the orbital angular momentum inside the nucleon vanishes. We also find that, despite the appeal of this solution of the ``nucleon spin crisis'', a solution where the axial anomaly is absent and its effects are traded for an appreciable strange quark polarization can not however be excluded --- because of the limited accuracy of the data --- unless this latter and/or the gluon polarization in the nucleon are explicitly measured.