Measurement of $\eta^\prime\rightarrow \pi^+\pi^- e^+e^-$ and $\eta^\prime\rightarrow \pi^+\pi^-\mu^+\mu^-$

TL;DR

This study measures the branching fractions of ta' decays to +-+- and +-+-, finding a precise value for the former and setting an upper limit for the latter, confirming theoretical predictions.

Contribution

The paper provides a high-precision measurement of the ta' to +- decay and sets a new upper limit for the ta' to +- decay, improving experimental constraints.

Findings

Branching fraction R(ta' to +-) = (2.11 b1 0.12 (stat.) b1 0.15 (syst.)) d7 10^{-3}

No ta' signal observed in +- spectrum

Upper limit for ta' to +- decay is < 2.9 d7 10^{-5} at 90% CL

Abstract

Based on a sample of 225.3 million J/\psi events accumulated with the BESIII detector at the BEPCII, the decays of \eta' to pi+pi-l+l- are studied via J/\psi to \gamma\eta'. A clear \eta' signal is observed in the pi+pi-e+e- mass spectrum, and the branching fraction is measured to be \BR(\eta' to pi+pi-e+e-) = (2.11\pm0.12 (stat.)\pm0.15 (syst.))\times10^{-3}, which is in good agreement with theoretical predictions and the previous measurement, but is determined with much higher precision. No \eta' signal is found in the pi+ pi- mu+ mu- mass spectrum, and the upper limit is determined to be \BR(\eta' to pi+ pi- mu+ mu-)<2.9\times10^{-5} at the 90% confidence level.