Parameterisation of [σ1/2−σ3/2] for Q2≥0 and non-resonance contribution to the GDH sum rule

A description of the virtual photon absorption cross section difference σ1/2−σ3/2 for the proton and neutron is obtained with a parameterisation based on a Regge type approach. The 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