Coherent Neutral Pion and eta Meson Photoproduction on the Deuteron

Coherent photoproduction of the neutral pion and eta meson on the deuteron, $$\gamma {d}$$ $$\rightarrow $$ $$\pi ^0\eta {d}$$ , has been experimentally studied at $$E_\gamma <1.15$$ GeV. The total cross section as a function of the incident energy shows a rapid increasing behavior below 1 GeV. The existing theoretical calculations well-reproduce the data including final-state interactions. We have measured the differential cross sections $$d\sigma /dM_{\pi d}$$ , $$d\sigma /dM_{\eta d}$$ , and $$d\sigma /d\Omega _d$$ in the $$\gamma d$$ center-of-mass frame for the first time. An enhancement is observed near the $$\eta d$$ threshold in $$d\sigma /dM_{\eta d}$$ , suggesting a strong $$\eta d$$ attraction. The enhancement is consistent with a predicted isoscalar $$1^-$$ $$\eta NN$$ bound state from the $$\eta NN$$ and $$\pi NN$$ coupled-channel calculation.