Search for the weak decay η ′ → K ± π ∓ and precise measurement of the branching fraction B ( J / ψ → ϕ η ′ )

We present the first search for the rare decay of ${\ensuremath{\eta}}^{\ensuremath{'}}$ into ${K}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$ in $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\phi}{\ensuremath{\eta}}^{\ensuremath{'}}$, using a sample of $1.3\ifmmode\times\else\texttimes\fi{}1{0}^{9}\text{ }\text{ }J/\ensuremath{\psi}$ events collected with the BESIII detector. No significant signal is observed, and the upper limit at the 90% confidence level for the ratio $\frac{\mathcal{B}({\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}{K}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}})}{\mathcal{B}({\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})}$ is determined to be $1.3\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}4}$. In addition, we report the measurement of the branching fraction of $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\phi}{\ensuremath{\eta}}^{\ensuremath{'}}$ to be $[5.10\ifmmode\pm\else\textpm\fi{}0.03(\text{stat})\ifmmode\pm\else\textpm\fi{}0.32(\text{syst})]\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}4}$, which agrees with previous results from BESII.