Is There Really a Spin Crisis?**The project supported in part by the NSF (19675018), SED and SSTD of China

Abstract

The matrix element of quark axial vector current is shown to be different from the nonrelativistic quark spin sum for a nucleon at rest. The nucleon spin content discovered in polarized deep inelastic scattering is shown to be accommodated in a constituent quark model with 15% sea quark component mixing. The relativistic correction and sea quark pair excitation inherently related to quark axial vector current reduce the nucleon axial charge and this reduction is compensated by the relativistic quark orbital angular momentum exactly and in turn keeps the nucleon spin untouched. Nucleon tensor charge has similar but smaller relativistic and sea quark pair excitation reduction.