Measurement of the Branching Fraction of and Search for a CP-Violating Asymmetry in $\hat{I}{\cdot }^{\prime }\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}{e}^{+}{e}^{-}$ at BESIII

Abstract

The rare decay $\hat{I}{\cdot }^{\prime }\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}{e}^{+}{e}^{-}$ is studied using a sample of $1.3\times 10^9$ $J/\ddot{I}ˆ$ events collected with the BESIII detector at BEPCII in 2009 and 2012. The branching fraction is measured with improved precision to be $\left(2.42\pm 0.05_{stat.}\pm 0.08_{syst.}\right)\times 10^{-3}$. 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 $\ddot{I}{€}^{+}\ddot{I}{€}^{-}$-pair and the $e^{+}{e}^{-}$-pair is investigated. A measurable value would indicate physics beyond the standard model; the result is ${\mathcal{A}}_{CP}=\left(2.9\pm 3.7_{stat.}\pm 1.1_{syst.}\right)\%$, which is consistent with the standard model expectation of no CP-violation. The precision is comparable to the asymmetry measurement in the $K_L^0\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}{e}^{+}{e}^{-}$ decay where the observed $(14\pm 2)\%$ effect is driven by a standard model mechanism.

10 pages, 8 figures, 3 tables