The spin structure of the nucleon

The spin structure of the nucleon has been extensively studied using deep inelastic polarized electron and muon scattering on polarized targets at CERN, DESY and SLAC. The data agree with the Bjorken sum rule, a fundamental prediction of QCD. A small fraction, about 20, of the nucleon spin seem to be carried by the quarks. However, this conclusion, may result from neglecting the contribution of the gluon spins. Future experiments will try to measure the unknown gluon polarization. The COMPASS project at CERN is discussed. (20 refs)