Observation of the doubly radiative decay η′→γγπ0

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Abstract

Based on a sample of 1.31 billion $J/\ensuremath{\psi}$ events collected with the BESIII detector, we report the study of the doubly radiative decay ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}{\ensuremath{\pi}}^{0}$ for the first time, where the ${\ensuremath{\eta}}^{\ensuremath{'}}$ meson is produced via the $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}^{\ensuremath{'}}$ decay. The branching fraction of ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}{\ensuremath{\pi}}^{0}$ inclusive decay is measured to be $\mathcal{B}({\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}{\ensuremath{\pi}}^{0}{)}_{\mathrm{Incl}}=\phantom{\rule{0ex}{0ex}}(3.20\ifmmode\pm\else\textpm\fi{}0.07(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.23(\mathrm{sys}))\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, while the branching fractions of the dominant process ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\omega}$ and the nonresonant component are determined to be $\mathcal{B}({\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\omega})\ifmmode\times\else\texttimes\fi{}\mathcal{B}(\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\pi}}^{0})=(23.7\ifmmode\pm\else\textpm\fi{}1.4(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}1.8(\mathrm{sys}))\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and $\mathcal{B}({\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}{\ensuremath{\pi}}^{0}{)}_{\mathrm{NR}}=(6.16\ifmmode\pm\else\textpm\fi{}0.64(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.67(\mathrm{sys}))\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, respectively. In addition, the ${M}_{\ensuremath{\gamma}\ensuremath{\gamma}}^{2}$-dependent partial widths of the inclusive decay are also presented.

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