The rare decay ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ is studied using a sample of $1.3\times {10}^{9}J/\psi$ events collected with the BESIII detector at BEPCII in 2009 and 2012. The branching fraction is measured with improved precision to be $(2.42\pm 0.05_{\mathrm{stat}}\pm 0.08_{\mathrm{syst}})\times {10}^{-3}$. Due to the inclusion of new data, this result supersedes the last BESIII result on this branching fraction. In addition, the $CP$-violating asymmetry in the angle between the decay planes of the ${\pi }^{+}{\pi }^{-}$-pair and the $e^{+}{e}^{-}$-pair is investigated. A measurable value would indicate physics beyond the standard model; the result is ${\mathcal{A}}_{CP}=(2.9\pm 3.7_{\mathrm{stat}}\pm 1.1_{\mathrm{syst}})\%$, which is consistent with the standard model expectation of no $CP$-violation. The precision is comparable to the asymmetry measurement in the $K_L^0\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ decay where the observed $(14\pm 2)\%$ effect is driven by a standard model mechanism.

• Received 17 November 2020
• Accepted 23 April 2021

DOI:https://doi.org/10.1103/PhysRevD.103.092005

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Published by the American Physical Society