First measurement using elliptically polarized photons of the double-polarization observable E for γ p → p π⁰ and $γ p → n π^+$

We report the measurement of the helicity asymmetry E for the pπ⁰ and nπ⁺ 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 pπ⁰ final state (230 - 842 MeV for the nπ⁺ final state) and the full range of pion polar angles, θ, providing the most precise measurement of the observable E. A moment analysis gives a clear observation of the pη cusp in the pπ⁰ final state.