Search for the weak decay $\eta^{\prime}\to K^{\pm}\pi^{\mp}$ and   precise measurement of the branching fraction ${\cal   B}(J/\psi\to\phi\eta^{\prime})$
  We present the first search for the rare decay of $\eta^\prime$ into $K^{\pm}\pi^{\mp}$ in $J/\psi\to\phi\eta^{\prime}$, using a sample of $1.3\times10^{9}$ $J/\psi$ events collected with the BESIII detector. No significant signal is observed, and the upper limit at the 90\% confidence level for the ratio $\frac{{\cal B}(\eta^{\prime}\to K^{\pm}\pi^{\mp})}{{\cal B}(\eta^{\prime}\to\gamma\pi^{+}\pi^{-})}$ is determined to be $1.3\times10^{-4}$. In addition, we report the measurement of the branching fraction of $J/\psi\to\phi\eta^{\prime}$ to be $(5.10\pm0.03(\text{stat.})\pm0.32(\text{syst.}))\times10^{-4}$, which agrees with previous results from BESII. 
BESIII Collaboration: M. Ablikim, M. N. Achasov, X. C. Ai, O.   Albayrak, M. Albrecht, D. J. Ambrose, A. Amoroso, F. F. An, ...
hep-ex