Measurement of the Branching Fraction of and Search for a CP-Violating Asymmetry in ${\eta }^{\mathrm{\prime }}\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ at BESIII

The rare decay ${\eta }^{\mathrm{\prime }}\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ is studied using a sample of $1.3\times {10}^9$ $J\mathrm{/}\psi$ events collected with the BESIII detector at BEPCII in 2009 and 2012. The branching fraction is measured with improved precision to be $(2.42\pm {0.05}_{stat\mathrm{.}}\pm {0.08}_{syst\mathrm{.}})\times {10}^{-3}$. 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}${CP} = \left(2.9\pm3.7{stat.}\pm1.1_{syst.} \right)%$, 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 $(14\pm 2)\mathrm{\%}$ effect is driven by a standard model mechanism.