Axial anomaly and the proton spin

Abstract

Several authors have noted that the forward matrix element of the axial-vector current as measured in polarized deep-inelastic electroproduction represents not only the quark contribution to the target proton spin but also the gluonic component due to the anomaly. We show that the anomalous divergence equations constrain the size of this gluonic contribution. In particular it is of such sign and magnitude that the resulting quark content of the proton spin deviates even further from the naive-quark-model expectations.