## Abstract

The Gerasimov-Drell-Hearn (GDH) sum rule connects the anomalous contribution to the magnetic moment of the target nucleus with an energy-weighted integral of the difference of the helicity-dependent photoabsorption cross sections. Originally conceived for real photons, the GDH integral can be generalised to the case of photons with virtuality Q^{2}. For spin-1/2 targets such as the nucleon, it then represents the non-perturbative limit of the first moment Γ_{1} of the spin structure function g_{1}(x, Q^{2}) in deep inelastic scattering (DIS). The data collected by HERMES with a deuterium target are presented together with a re-analysis of previous measurements on the proton. This provides an unprecedented and complete measurement of the generalised GDH integral for photon-virtuality ranging over 1.2 < Q^{2} < 12.0 GeV^{2} and for photon-nucleon invariant mass squared W^{2} ranging over 1 < W^{2} < 45 GeV^{2}, thus covering simultaneously the nucleon-resonance and the deep inelastic scattering regions. These data allow the study of the Q^{2}-dependence of the full GDH integral, which is sensitive to both the Q^{2}-evolution of the resonance form factors and contributions of higher twist. The contribution of the nucleon-resonance region is seen to decrease rapidly with increasing Q^{2}. The DIS contribution is sizeable over the full measured range, even down to the lowest measured Q^{2}. As expected, at higher Q^{2} the data are found to be in agreement with previous measurements of the first moment of g_{1}. From data on the deuteron and proton, the GDH integral for the neutron has been derived and the proton-neutron difference evaluated. This difference is found to satisfy the fundamental Bjorken sum rule at Q^{2} = 5 GeV^{2}.

Original language | English (US) |
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Pages (from-to) | 527-538 |

Number of pages | 12 |

Journal | European Physical Journal C |

Volume | 26 |

Issue number | 4 |

DOIs | |

State | Published - Feb 2003 |

## ASJC Scopus subject areas

- Engineering (miscellaneous)
- Physics and Astronomy (miscellaneous)

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