Search for the weak decay ${\eta }^{\prime }\to K^{\pm }{\pi }^{\mp }$ and precise measurement of the branching fraction $\mathcal{B}(J/\psi \to \varphi {\eta }^{\prime })$

We present the first search for the rare decay of ${\eta }^{\prime }$ into $K^{\pm }{\pi }^{\mp }$ in $J/\psi \to \varphi {\eta }^{\prime }$, using a sample of $1.3\times 10^{9}J/\psi$ events collected with the BESIII detector. No significant signal is observed, and the upper limit at the 90% confidence level for the ratio $\frac{\mathcal{B}({\eta }^{\prime }\to K^{\pm }{\pi }^{\mp })}{\mathcal{B}({\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-})}$ is determined to be $1.3\times 10^{-4}$. In addition, we report the measurement of the branching fraction of $J/\psi \to \varphi {\eta }^{\prime }$ to be $[5.10\pm 0.03(\text{stat})\pm 0.32(\text{syst})]\times 10^{-4}$, which agrees with previous results from BESII.

Figure 1

Scatter plot of $M(\gamma {\pi }^{+}{\pi }^{-})$ versus $M(K^{+}{K}^{-})$.

Figure 2

Distributions of (a) $M(K^{+}{K}^{-})$ and (b) $M(\gamma {\pi }^{+}{\pi }^{-})$ with projections of the fit result superimposed for $J/\psi \to \varphi {\eta }^{\prime }$, $\varphi \to K^{+}{K}^{-}$, ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$. The dots with errors are for data, the solid curve shows the result of the fit to signal plus background distributions, the long-dashed curve is for the $\varphi {\eta }^{\prime }$ signal, the dot-dashed curve shows the non-${\eta }^{\prime }$-peaking background, the dotted curve shows the non-$\varphi$-peaking background, and the short-dashed curve is for the non-$\varphi {\eta }^{\prime }$ background.

Figure 3

(a) Scatter plot of $M(K^{+}{K}^{-})$ versus $M(K^{\pm }{\pi }^{\mp })$, where the box indicates the signal region with $|M(K^{+}{K}^{-})-M(\varphi )|<15\mathrm{MeV}/c^2$ and $|M(K^{\pm }{\pi }^{\mp })-M({\eta }^{\prime })|<7\mathrm{MeV}/c^2$. (b) The $K^{\pm }{\pi }^{\mp }$ invariant mass distribution, where the arrows show the signal region. The dots with error bars are for data, the dashed histogram is for the signal MC with arbitrary normalization, and the solid histogram is the background contamination from a MC simulation of $J/\psi \to \varphi {\pi }^{+}{\pi }^{-}$.