Search for intermediate resonances and dark gauge bosons in J/ψ→γπ0η′

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We report on an analysis of the decay $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\pi}}^{0}{\ensuremath{\eta}}^{\ensuremath{'}}$ using a sample of $(1310.6\ifmmode\pm\else\textpm\fi{}7.0)\ifmmode\times\else\texttimes\fi{}{10}^{6}J/\ensuremath{\psi}$ events collected with the BESIII detector. We search for the $CP$-violating process ${\ensuremath{\eta}}_{c}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\eta}}^{\ensuremath{'}}$ and a dark gauge boson ${U}^{\ensuremath{'}}$ in $J/\ensuremath{\psi}\ensuremath{\rightarrow}{U}^{\ensuremath{'}}{\ensuremath{\eta}}^{\ensuremath{'}},{U}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\pi}}^{0},{\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$. No evidence of an ${\ensuremath{\eta}}_{c}$ signal is observed in the ${\ensuremath{\pi}}^{0}{\ensuremath{\eta}}^{\ensuremath{'}}$ invariant-mass spectrum and the upper limit of the branching fraction is determined to be $5.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ at the 90% confidence level. We also find no evidence of ${U}^{\ensuremath{'}}$ production and set upper limits at the 90% confidence level on the product branching fraction $\mathcal{B}(J/\ensuremath{\psi}\ensuremath{\rightarrow}{U}^{\ensuremath{'}}{\ensuremath{\eta}}^{\ensuremath{'}})\ifmmode\times\else\texttimes\fi{}\mathcal{B}({U}^{\ensuremath{'}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma})$ in the range between $(0.8\ensuremath{-}6.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ for $0.2\ensuremath{\le}{m}_{{U}^{\ensuremath{'}}}\ensuremath{\le}2.1\text{ }\text{ }\mathrm{GeV}/{c}^{2}$. In addition, we study the process $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\omega}{\ensuremath{\eta}}^{\ensuremath{'}}$ with $\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\pi}}^{0}$. The branching fraction of $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\omega}{\ensuremath{\eta}}^{\ensuremath{'}}$ is found to be $(1.87\ifmmode\pm\else\textpm\fi{}0.09\ifmmode\pm\else\textpm\fi{}0.12)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, where the first uncertainty is statistical and the second is systematic, with a precision that is improved by a factor of 1.4 over the previously published BESIII measurement.

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