Search for intermediate resonances and dark gauge bosons in $J/\psi \to \gamma {\pi }^0{\eta }^{\prime }$

Abstract

We report on an analysis of the decay $J/\psi \to \gamma {\pi }^0{\eta }^{\prime }$ using a sample of $(1310.6\pm 7.0)\times 10^{6}J/\psi$ events collected with the BESIII detector. We search for the CP-violating process ${\eta }_c\to {\pi }^0{\eta }^{\prime }$ and a dark gauge boson $U^{\prime }$ in $J/\psi \to U^{\prime }{\eta }^{\prime },U^{\prime }\to \gamma {\pi }^0,{\pi }^0\to \gamma \gamma$. No evidence of an ${\eta }_c$ signal is observed in the ${\pi }^0{\eta }^{\prime }$ invariant-mass spectrum and the upper limit of the branching fraction is determined to be $7.2\times 10^{-5}$ at the 90\% confidence level. We also find no evidence of $U^{\prime }$ production and set upper limits at the 90\% confidence level on the product branching fraction $\mathcal{B}(J/\psi \to U^{\prime }{\eta }^{\prime })\times \mathcal{B}(U^{\prime }\to {\pi }^0\gamma )$ in the range between $(0.8-6.5)\times 10^{-7}$ for 0.2 $\le m_{U^{\prime }}\le 2.1$GeV$/c^2$. In addition, we study the process $J/\psi \to \omega {\eta }^{\prime }$ with $\omega \to \gamma {\pi }^0$. The branching fraction of $J/\psi \to \omega {\eta }^{\prime }$ is found to be $(1.87\pm 0.09\pm 0.12)\times 10^{-4}$, 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.