Observation of $\psi \left(3686\right)\to p\overline{p}{\eta }^{\prime }$ and improved measurement of $J/\psi \to p\overline{p}{\eta }^{\prime }$

We observe the process $\psi \left(3686\right)\to p\overline{p}{\eta }^{\prime }$ for the first time, with a statistical significance higher than $10\sigma $, and measure the branching fraction of $J/\psi \to p\overline{p}{\eta }^{\prime }$ with an improved accuracy compared to earlier studies. The measurements are based on $4.48×{10}^{8}\text{}\text{}\psi \left(3686\right)$ and $1.31×{10}^{9}\text{}\text{}J/\psi $ events collected by the BESIII detector operating at the BEPCII. The branching fractions are determined to be $B\left(\psi \left(3686\right)\to p\overline{p}{\eta }^{\prime }\right)=\left(1.10±0.10±0.08\right)×{10}^{-5}$ and $B\left(J/\psi \to p\overline{p}{\eta }^{\prime }\right)=\left(1.26±0.02±0.07\right)×{10}^{-4}$, where the first uncertainties are statistical and the second ones systematic. Additionally, the $\eta -{\eta }^{\prime }$ mixing angle is determined to be $-24°±11°$ based on $\psi \left(3686\right)\to p\overline{p}{\eta }^{\prime }$, and $-24°±9°$ based on $J/\psi \to p\overline{p}{\eta }^{\prime }$, respectively.