STATUS OF THE PROTON SPIN PROBLEM

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Abstract
The proton spin problem triggered by the EMC experiment and its present status are closely examined. Recent experimental and theoretical progresses and their implications are reviewed. It is pointed out that the sign of the sea quark polarization generated perturbatively by hard gluons via the anomaly mechanism is predictable. It is negative if the gluon spin component is positive. We stress that the polarized nucleon structure function g1(x) is independent of the k⊥ factorization scheme chosen in defining the quark spin density and the hard photon–gluon scattering cross-section. Consequently, the anomalous gluon and sea quark interpretations for Γ1, the first moment of g1(x), are equivalent. It is the axial anomaly that accounts for the observed suppression of .

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