标题： 首次使用椭圆偏振光子测量双极化可观测量$E$对于$γp \to p π^0$和$γp \to n π^+$ 显示英文标题

标题： First measurement using elliptically polarized photons of the double-polarization observable $E$ for $γp \to p π^0$ and $γp \to n π^+$

摘要： 我们报告了在MAMI的Crystal Ball实验中，首次结合椭圆偏振光子束和纵向极化靶测量了螺旋度不对称性$E$对于$p\pi^0$和$n\pi^+$最终态的结果。结果与使用圆偏振光子束获得的数据非常吻合，表明可以同时测量需要线性偏振光子（例如$G$）和圆偏振光子（例如$E$）以及纵向极化靶的极化可观测量。 新的数据覆盖了光子能量范围270 - 1400 MeV的$p\pi^0$最终态（230 - 842 MeV的$n\pi^+$最终态）以及介子极角$\theta$的全部范围，提供了对可观测量$E$的最精确测量。 动量分析在$p\pi^0$最终态中清晰地观察到了$p\eta$的尖点。 显示英文摘要

摘要： We report the measurement of the helicity asymmetry $E$ for the $p\pi^0$ and $n\pi^+$ 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 $p\pi^0$ final state (230 - 842 MeV for the $n\pi^+$ final state) and the full range of pion polar angles, $\theta$, providing the most precise measurement of the observable $E$. A moment analysis gives a clear observation of the $p\eta$ cusp in the $p\pi^0$ final state.