Observation of ?? ? ?+?-?+?-

Abstract

Using (1310.6 +/- 7.0) x 10(6) J/psi events acquired with the BESIII detector at the BEPCII storage rings, the decay eta' -> pi(+)pi(-)mu(+)mu(-) is observed for the first time with a significance of 8 sigma via the process J/psi -> gamma eta'. We measure the branching fraction of eta' -> pi(+)pi(-)mu(+)mu(-) to be B(eta' -> pi(+)pi(-)mu(+)mu(-)) = (1.97 +/- 0.33(stat) +/- 0.19(syst)) x 10(-5), where the first and second uncertainties are statistical and systematic, respectively.