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