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

We observe the process $\psi (3686)\to p\overline{p}{\eta }^{\prime }$ 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 $4.48\times {10}^8\psi (3686)$ and $1.31\times {10}^{9}J/\psi$ events collected by the BESIII detector operating at the BEPCII. The branching fractions are determined to be $\mathcal{B}(\psi (3686)\to p\overline{p}{\eta }^{\prime })=(1.10\pm 0.10\pm 0.08)\times {10}^{-5}$ and $\mathcal{B}(J/\psi \to p\overline{p}{\eta }^{\prime })=(1.26\pm 0.02\pm 0.07)\times {10}^{-4}$, where the first uncertainties are statistical and the second ones systematic. Additionally, the $\eta -{\eta }^{\prime }$ mixing angle is determined to be $-24°\pm 11°$ based on $\psi (3686)\to p\overline{p}{\eta }^{\prime }$, and $-24°\pm 9°$ based on $J/\psi \to p\overline{p}{\eta }^{\prime }$, respectively.

Figure 1

Invariant mass spectra of the ${\eta }^{\prime }$ candidates in $\psi (3686)\to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$ (a), $\psi (3686)\to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}$ (b), $J/\psi \to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$ (c), and $J/\psi \to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}$ (d). The dots with error bars are data, the shaded histograms are the backgrounds from inclusive MC samples, the blue solid curves are the fit results, and the red dashed curves are the backgrounds from fit.

Figure 2

Dalitz plots of $\psi (3686)\to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$ (a), $\psi (3686)\to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}$ (b), $J/\psi \to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$ (c), and $J/\psi \to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}$ (d).

Figure 3

Invariant mass distributions of $p{\eta }^{\prime }$ (a), $\overline{p}{\eta }^{\prime }$ (b), and $p\overline{p}$ (c) for $\psi (3686)\to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$, and those of $p{\eta }^{\prime }$ (d), $\overline{p}{\eta }^{\prime }$ (e), and $p\overline{p}$ (f) for $\psi (3686)\to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$. The dots with error bars are data with background subtracted, and the red lines are the corresponding signal MC.

Figure 4

Invariant mass distributions of $p{\eta }^{\prime }$ (a), $\overline{p}{\eta }^{\prime }$ (b), and $p\overline{p}$ (c) for $J/\psi \to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$, and those of $p{\eta }^{\prime }$ (d), $\overline{p}{\eta }^{\prime }$ (e), and $p\overline{p}$ (f) for $J/\psi \to p\overline{p}{\eta }^{\prime }$ with ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$. The dots with error bars show background subtracted data, and the red lines are the corresponding distributions from signal MC.