Formation of the pseudoscalarsπ0,η, and η’ in the reactionγγ→γγ

The Crystal Ball detector has been used at the DORIS II storage ring at DESY to study the reaction ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$→${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$γ*γ*→${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$γγ, where X is a narrow resonance with mass between 100 and 3000 MeV. Formation of ${\pi }^0$, η, and η’ mesons is observed, and the following meson partial widths are obtained: ${\mathrm{\Gamma }}_{\mathrm{\pi }0\mathrm{\Gamma }0\to \gamma \gamma }$=7.7±0.5±0.5 eV, ${\mathrm{\Gamma }}_{\mathrm{\eta }\mathrm{-}\gamma \gamma }$=0.514±0.017±0.035 keV, and ${\mathrm{\Gamma }}_{\mathrm{\eta }\mathcal{’}\to \gamma \gamma }$=4.7±0.5±0.5 keV. No other narrow resonances are observed, and upper limits are given for the product ${\Gamma }_{X\to \gamma \gamma }$ $B_{X\to \gamma \gamma }$. .AE