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

We observe the process $\psi (3686)\to p\bar{p}{\eta }^{\mathrm{\prime }}$ for the first time, with a statistical significance higher than 10$\sigma$, and measure the branching fraction of $J\mathrm{/}\psi \to p\bar{p}{\eta }^{\mathrm{\prime }}$ with an improved accuracy compared to earlier studies. The measurements are based on $4.48 \times 10^8$ $\psi (3686)$ and $1.31\times {10}^9$ $J\mathrm{/}\psi$ events collected by the BESIII detector operating at the BEPCII. The branching fractions are determined to be $B(\psi(3686) \to p \bar{p} \eta^{\prime}) = (1.10\pm0.10\pm0.08)\times10^{-5}$and$B(J/\psi \to p \bar{p} \eta^{\prime})=(1.26\pm0.02\pm 0.07)\times10^{-4}$, where the first uncertainties are statistical and the second ones systematic. Additionally, the $\eta -{\eta }^{\mathrm{\prime }}$ mixing angle is determined to be $-24^{\circ} \pm 11^{\circ}$basedon$\psi(3686) \to p \bar{p} \eta^{\prime}$,and$-24^{\circ} \pm 9^{\circ}$basedon$J/\psi \to p \bar{p} \eta^{\prime}$, respectively.