Determination of the deep inelastic contribution to the generalised Gerasimov-Drell-Hearn integral for the proton and neutron

The virtual photon absorption cross section differences [ σ 1/2 − σ 3/2 ] for the proton and neutron have been determined from measurements of polarised cross section asymmetries in deep inelastic scattering of 27.5 GeV longitudinally polarised positrons from polarised 1 H and 3 He internal gas targets. The data were collected in the region above the nucleon resonances in the kinematic range ν < 23.5 GeV and 0.8 GeV 2 < Q 2 < 12 GeV 2 . For the proton the contribution to the generalised Gerasimov-Drell-Hearn integral was found to be substantial and must be included for an accurate determination of the full integral. Furthermore the data are consistent with a QCD next-to-leading order fit based on previous deep inelastic scattering data. Therefore higher twist effects do not appear significant.