Measurement of eta ' -> pi(+)pi(-)e(+)e(-) and eta ' -> pi(+)pi(-)mu(+)mu(-)

Based on a sample of 225.3 million J/psi events accumulated with the BESIII detector at the BEPCII, the decays of eta' -> pi(+)pi(-) l(+)l(-) are studied via J/psi -> gamma eta'. A clear eta' signal is observed in the pi(+)pi(-) e(+)e(-) mass spectrum, and the branching fraction is measured to be B(eta' -> pi(+)pi(-) e(+)e(-)) = (2.11 +/- 0.12(stat) +/- 0.14(syst)) x 10(-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 B(eta' -> pi(+)pi(-) mu(+)mu(-)) <2.9 x 10(-5) at the 90% confidence level.