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

TL;DR

This paper reports a precise measurement of the rare decay $ ext{eta'} o ext{pi}^+ ext{pi}^- e^+ e^-$ branching fraction and searches for CP violation, finding results consistent with the standard model and improving previous measurements.

Contribution

The study provides the most precise measurement to date of the $ ext{eta'}$ decay branching fraction and investigates CP violation with new data, setting constraints on new physics.

Findings

Branching fraction measured as (2.42 ± 0.05(stat) ± 0.08(syst)) × 10^{-3}

CP asymmetry measured as (2.9 ± 3.7(stat) ± 1.1(syst))%

Results are consistent with standard model predictions.

Abstract

The rare decay $\eta' \to \pi^+ \pi^- e^+ e^-$ 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 $\left( 2.42\pm0.05_{stat.}\pm0.08_{syst.} \right) \times 10^{-3}$. 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 $\pi^+\pi^-$-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 $K^0_L\to\pi^+\pi^-e^+e^-$ decay where the observed $(14\pm2)\%$ effect is driven by a standard model mechanism.