[en] The structure of the nucleon has many important features that are yet to be uncovered. Of current interest is the nucleon spin-structure which can be measured by doing double-polarization experiments with photon beams of medium energies (0.1 to 2 GeV). One such experiment uses dispersion relations, applied to the Compton scattering amplitude, to relate measurement of the total reaction cross section integrated over the incident photon energy to the nucleon anomalous magnetic moment. At present, no single facility spans the entire range of photon energies necessary to test this sum rule. The Laser-Electron Gamma Source (LEGS) facility will measure the double-polarization observables at photon energies between 0.15-0.47 MeV. Either the SPring-8 facility, the GRAAL facility (France). or Jefferson Laboratory could make similar measurements at higher photon energies. (author)

[en] Neutral pion photoproduction from a liquid deuterium target was measured in the energy region near 300 MeV at the LEGS facility of Brookhaven National Laboratory. The inclusive cross sections from deuterium are in agreement with measurements from Mainz, yet the exclusive cross sections and spin asymmetries for neutral pion production in coincidence with a detected nucleon are much smaller than expected from a quasi-free approximation. This may indicate that substantial final state interactions play a significant role, which will complicate the extraction of the desired amplitudes that would be measured if a free neutron target could be used. (author)

[en] The structure of the nucleon has many important features that are yet to be uncovered. Of current interest is the nucleon spin-structure which can be measured by doing double-polarization experiments with photon beams of medium energies (0.1 to 2 GeV). One such experiment uses dispersion relations, applied to the Compton scattering amplitude, to relate measurement of the total reaction cross section integrated over the incident photon energy to the nucleon anomalous magnetic moment. At present, no single facility spans the entire range of photon energies necessary to test this sum rule. The Laser-Electron Gamma Source (LEGS) facility will measure the double-polarization observables at photon energies between 0.15--0.47 MeV. Either the SPring8 facility, the GRAAL facility (France), or Jefferson Laboratory could make similar measurements at higher photon energies. A high-precision measurement of the spin-polarizability and the Drell-Hearn-Gerasimov sum rule is now possible with the advent of high-polarization solid HD targets at medium energy polarized photon facilities such as LEGS, GRAAL and SPring8. Other facilities with lower polarization in either the photon beam or target (or both) are also pursuing these measurements because of the high priority associated with this physics. The Spin-asymmetry (SASY) detector that will be used at LEGS has been briefly outlined in this paper. The detector efficiencies have been explored with simulations studies using the GEANT software, with the result that both charged and uncharged pions can be detected with a reasonable efficiency (> 30%) over a large solid angle. Tracking with a TPC, which will be built at LEGS over the next few years, will improve the capabilities of these measurements

[en] A solid, polarized HD target has been developed for the measurement of double-polarization observables in the Δ resonance region. We report here the inaugural data obtained with this target. This new polarized target technology, combined with the high quality LEGS photon beam and the large acceptance spectrometer, SASY, provides a unique facility for studying the spin structure of the nucleon below 500 MeV. Pion production data collected on a longitudinally polarized target with six γ-ray polarization states provides the first simultaneous measurement of dσ/dΩ, as well as the Σ, G and E asymmetries. With the future addition of magnetic analysis to SASY, a complete set of pion production observables on the proton and the deuteron (neutron) will be obtained. (author)

[en] We report preliminary results of π photo-production using polarized γ beams and a polarized HD target. Four observables can be extracted simultaneously from the data, the cross section, the beam asymmetry Σ, and the double-polarization observables G and E. The latter determines the GDH sum rule integral

[en] We report new measurements of inclusive π production from frozen-spin HD for polarized photon beams covering the Δ(1232) resonance. These provide data simultaneously on both H and D with nearly complete angular distributions of the spin-difference cross sections entering the Gerasimov-Drell-Hearn (GDH) sum rule. Recent results from Mainz and Bonn exceed the GDH prediction for the proton by 22 μb, suggesting as yet unmeasured high-energy components. Our π⁰ data reveal a different angular dependence than assumed in Mainz analyses and integrate to a value that is 18 μb lower, suggesting a more rapid convergence. Our results for deuterium are somewhat lower than published data, considerably more precise, and generally lower than available calculations.

[en] 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.

[en] The extraction of resonance parameters from meson photo-reaction data is a challenging effort, that would greatly benefit from the availability of several polarization observables, measured for each reaction channel on both proton and neutron targets. In the aim of obtaining such complete experiments, polarized photon beams and targets have been developed at facilities, worldwide. We report on the latest results from the LEGS and GRAAL collaborations, providing single and double polarization measurements on pseudo-scalar meson photo-production from the nucleon.