The proton spin puzzle: A status report

The proton spin puzzle, inspired by the EMC experiment, and its present sta tus 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)over ba r> factorization scheme are clarified. It is stressed that the polarized nu cleon structure function g(1)(x) is independent of the factorization scheme chosen in defining the quark spin density. Consequently, the anomalous glu on and sea-quark interpretations for the deviation of the observed first mo ment of g(1)(p)(x) from the Ellis-Jaffe sum rule are equivalent. While it i s well known that the total quark spin in the chiral-invariant (CI) factori zation 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 l arge enough, it is much less known that, contrary to the gauge-invariant sc heme (e.g., the <(MS)over bar> scheme), the quark orbital angular momentum in the CI scheme deviates even farther from the relativistic quark model pr ediction. Recent developments in the NLO analysis of polarized DIS data, or bital angular momentum and lattice calculations of the proton spin content are briefly sketched.