The Q^2-dependence of the Generalised Gerasimov-Drell-Hearn Integral for the Proton

TL;DR

This study measures how the generalized Gerasimov-Drell-Hearn integral for the proton varies with Q^2, revealing dominance of deep-inelastic contributions at higher Q^2 and no significant deviations from expected 1/Q^2 behavior.

Contribution

First measurement of the Q^2 dependence of the generalized GDH integral for the proton across a broad range, separating resonance and deep-inelastic contributions.

Findings

Deep-inelastic region dominates for Q^2 > 3 GeV^2

Total integral follows 1/Q^2 behavior with no large resonance effects

Resonance and deep-inelastic contributions are both significant at low Q^2

Abstract

The dependence on Q^2 (the negative square of the 4-momentum of the exchanged virtual photon) of the generalised Gerasimov-Drell-Hearn integral for the proton has been measured in the range 1.2 GeV^2 < Q^2 < 12 GeV^2 by scattering longitudinally polarised positrons on a longitudinally polarised hydrogen gas target. The contributions of the nucleon-resonance and deep-inelastic regions to this integral have been evaluated separately. The latter has been found to dominate for Q^2 > 3 GeV^2, while both contributions are important at low Q^2. The total integral shows no significant deviation from a 1/Q^2 behaviour in the measured Q^2 range, and thus no sign of large effects due to either nucleon-resonance excitations or non-leading twist.