Recent Polarization Experiments and the GDH Sum Rule

Measurements of the spin-difference cross sections entering the Gerasimov-Drell-Hearn (GDH) sum rule are reviewed. Results on the proton from Mainz and Bonn exceeded the GDH prediction by 22 μb, requiring as yet unmeasured canceling high-energy components. Recent experiments with frozen-spin HD at BNL reveal a different angular dependence for π0 production than what was assumed in Mainz analyses in lieu of direct measurements and integrate to a value that is 18 μb lower, suggesting a rapid convergence. Results for deuterium over limited energy ranges are consistent with large canceling contributions but differ from existing state of the art calculations.