First Measurement Of The Helicity Dependence For The Gamma P -> P Pi(+)Pi(-) Reaction

The helicity dependence of the total cross-section and the invariant-mass distributions in the (p pi(+/-)) and (pi(+) pi(-)) final states for the gamma p -> p pi(+)pi(-) 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 Delta pi state, both the measured unpolarized and the helicity-dependent observables are generally not well described by the existing theoretical models.