The Proton Spin Puzzle: A Status Report
  The proton spin puzzle inspired by the EMC experiment and its present status are closely examined. Recent experimental progress is reviewed. Various factorization schemes due to the ambiguity arising from the axial anomaly are discussed. Some misconceptions in the literature about the $\MS$ factorization scheme are clarified. It is stressed that the polarized nucleon structure function $g_1(x)$ is independent of the factorization scheme chosen in defining the quark spin density. Consequently, the anomalous gluon and sea-quark interpretations for the deviation of the observed first moment of $g_1^p(x)$ from the Ellis-Jaffe sum rule are equivalent. While it is well known that the total quark spin in the chiral-invariant (CI) factorization scheme (e.g. the improved parton model) can be made to be close to the quark model expectation provided that the gluon spin is positive and large enough, it is much less known that, contrary to the gauge-invariant scheme (e.g. the MSbar scheme), the quark orbital angular momentum in the CI scheme deviates even farther from the relativistic quark model prediction. Recent developments in the NLO analysis of polarized DIS data, orbital angular momentum and lattice calculations of the proton spin content are briefly sketched. 
Hai-Yang Cheng