Measurement of the G asymmetry for the gamma p -> N pi channels in the Delta(1232) resonance region

The G asymmetry of the γp↦Nπ reaction has been measured for the first time for E γ = 340±14 MeV. This observable, for which very scarce published data exist, plays an important role to disentangle the contributions of the various nucleon resonances. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a linearly polarized, tagged photon beam, and a longitudinally polarized proton target

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