Observation of $η_{c}(2S) \to K^{+}K^{-}η$
By analyzing $(27.12 \pm 0.14)\times10^{8}$ $ψ(3686)$ events accumulated with the BESIII detector, the decay $η_{c}(2S) \to K^{+} K^{-} η$ is observed for the first time with a significance of $6.2σ$ after considering systematic uncertainties. The product of the branching fractions of $ψ(3686) \to γη_{c}(2S)$ and $η_{c}(2S) \to K^{+} K^{-} η$ is measured to be $\mathcal{B}(ψ(3686) \toγη_{c}(2S))\times \mathcal{B}(η_{c}(2S)\to K^{+} K^{-}η)=(2.39 \pm 0.32 \pm 0.34) \times 10^{-6}$, where the first uncertainty is statistical, and the second one is systematic. The branching fraction of $η_{c}(2S)\to K^{+} K^{-}η$ is determined to be $\mathcal{B}(η_{c}(2S)\to K^{+} K^{-}η) = (3.42 \pm 0.46 \pm 0.48 \pm 2.44) \times 10^{-3}$, where the third uncertainty is due to the branching fraction of $ψ(3686) \to γη_{c}(2S)$. Using a recent BESIII measurement of $\mathcal{B} (η_{c}(2S) \to K^{+} K^{-}π^{0})$, we also determine the ratio between the branching fractions of $η_{c}(2S) \to K^{+} K^{-}η$ and $η_{c}(2S) \to K^{+} K^{-}π^{0}$ to be $1.49 \pm 0.22 \pm 0.25$, which is consistent with the previous result of BaBar at a comparable precision level.