Observation of a structure at 1.84 GeV/c(2) in the 3(pi(+)pi(-)) mass spectrum in J/psi -> gamma 3(pi(+) pi(-)) decays

With a sample of 225.3 x 10(6) J/psi events taken with the BESIII detector, the decay J/psi -> gamma 3(pi(+) pi(-)) is analyzed. A structure at 1.84 GeV/c(2) is observed in the 3(pi(+) pi(-)) invariant mass spectrum with a statistical significance of 7.6 sigma. The mass and width are measured to be M = 1842.2 +/- 4.2(-2.6)(+7.1) MeV/c(2) and Gamma = 83 +/- 14 +/- 11 MeV. The product branching fraction is determined to be B(J/psi -> gamma X(1840) x B(X(1840) -> 3(pi(+)pi(-))) = (2.44 +/- 0.36(-0.74)(+0.60)) x 10(-5). No eta' signals are observed in the 3(pi(+) pi(-)) invariant mass spectrum, and the upper limit of the branching fraction for the decay eta' -> 3(pi(+) pi(-)) is set to be 3.1 x 10(-5) at a 90% confidence level.