First measurement using elliptically polarized photons of the double-polarization observable $E$ for ${\hat{I}}^{3}p\to p\ddot{I}{€}^0$ and ${\hat{I}}^{3}p\to n\ddot{I}{€}^{+}$

Abstract

We report the measurement of the helicity asymmetry $E$ for the $p\ddot{I}{€}^0$ and $n\ddot{I}{€}^{+}$ 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 $p\ddot{I}{€}^0$ final state (230 - 842 MeV for the $n\ddot{I}{€}^{+}$ final state) and the full range of pion polar angles, $\hat{I}¸$, providing the most precise measurement of the observable $E$. A moment analysis gives a clear observation of the $p\hat{I}\cdot$ cusp in the $p\ddot{I}{€}^0$ final state.