E. Thomas

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

Abstract A description of the virtual photon absorption cross section difference σ 1/2 −σ 3/2 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 1 H, 3 He and 2 H targets in the range 0.3 GeV 2 Q 2 2 and 4 GeV 2 W 2 2 . The fits give a reliable description of the data and provide predictions for the photo-production through a smooth Q 2 -transition. The contribution above the resonance region to the Gerasimov–Drell–Hearn sum rule for real and virtual photons has been evaluated. For the real photons this contribution accounts for a large fraction of the discrepancy between the sum rule expectations and the single pion photo-production analysis estimates.