First Measurement Using Elliptically Polarized Photons of the Double-Polarization Observable E for p- pit0 and p- nit plus

We report the measurement of the helicity asymmetry E for the pa-degrees and na-+ final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g., G) and circularly polarized photons (e.g., E) and a longitudinally polarized target. The new data cover a photon energy range 270-1400 MeV for the pa-degrees final state (230-842 MeV for the na-+ final state) and the full range of pion polar angles, 0, providing the most precise measurement of the observable E. A moment analysis gives a clear observation of the pri cusp in the pa-degrees final state.