First measurement of the helicity dependence for the gamma-p --> ppi+ pi- reaction

The helicity dependence of the total cross-section and the invariant-mass distributions in the (p±) and (+−) final states for the p → p+− reaction have been measured for the first time at incident photon energies from 400 to 800MeV. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Although this channel is found to be predominantly excited by the intermediate production of a state, both the measured unpolarized and the helicity-dependent observables are generally not well described by the existing theoretical models

Measrurement of Helicity-Dependent Photoabsorption Cross Sections on the Neutron from 815 MeV to 1825 MeV

Helicity-dependent total photoabsorption cross sections on the deuteron have been measured for the first time at ELSA (Bonn) in the photon energy range from 815 to 1825 MeV. Circularly polarized tagged photons impinging on a longitudinally polarized LiD target have been used together with a highly efficient 4 detector system. The data around 1 GeV are not compatible with predictions from existing multipole analyses. From the measured energy range an experimental contribution to the GDH integral on the neutron of [33.9 +- 5.5stat 4.5syst] micro barn is extracted