A measurement of the spin asymmetry and determination of the structure function g1 in deep inelastic muon-proton scattering

The spin asymmetry in deep inelastic scattering of longitudinally polarised muons by longitudinally polarised protons has been measured over a large x range (0.01< x<0.7). The spin-dependent structure function g₁( x) for the proton has been determined and its integral over x found to be 0.114±0.012±0.026, in disagreement with the Ellis-Jaffe sum rule. Assuming the validity of the Bjorken sum rule, this result implies a significant negative value for the integral of g₁ for the neutron. These values for the integrals of g₁ lead to the conclusion that the total quark spin constitutes a rather small fraction of the spin of the nucleon.