Measurement of neutral pion photoproduction off the proton with the large acceptance electromagnetic calorimeter BGOegg

Abstract

Differential cross sections and photon beam asymmetries for the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}p$ have been measured in the photon beam energy range of $1.3--2.4\phantom{\rule{4pt}{0ex}}\mathrm{GeV}$ and the ${\ensuremath{\pi}}^{0}$ polar angle range of $\ensuremath{-}1<cos{\ensuremath{\theta}}_{{\ensuremath{\pi}}^{0}}^{\mathrm{c}.\mathrm{m}.}<0.7$. The measurement has been done by using a large acceptance electromagnetic calorimeter in the SPring-8 LEPS2/BGOegg experiment. The results of the photon beam asymmetry in a wide ${\ensuremath{\pi}}^{0}$ polar angle range are new for the photon beam energies exceeding $1.9\phantom{\rule{4pt}{0ex}}\mathrm{GeV}$. None of the existing partial wave analysis models describe the observed asymmetries in the high energy region. The contribution of multipole amplitudes with an orbital angular momentum of 5 in the $\ensuremath{\pi}N$ system has been found to be important at these energies.