Non-resonance contribution to GDH sum rule

A parametrisation of the virtual photon absorption cross section difference [σ1/2 - σ3/2] for the proton and neutron is presented. The Regge inspired parametrisation is obtained from global fits to the cross section data derived from the spin asymmetries measured in deep inelastic scattering of longitudinally polarised leptons from polarised 1H, 3He and 2H targets in the range 0.3 GeV2 < Q2 < 70 GeV2 and 4 GeV2 < W2 < 300 GeV2 . An accurate description of the data is provided and a reliable prediction for the photo- production through a smooth Q2-transition is given. The parametrisation is used to evaluate the contribution above the resonance region to the Gerasimov-Drell-Hearn sum rule for real and virtual photons. The main result is the large high-energy and multi-hadron contribution to the GDH sum rule, particularly in the isovector channel. Taking into account this large contribution, together with previous estimates of single-pion and resonance contributions, the GDH sum rule seems to be fulfilled. (author)