Observation of a structure at 1.84 GeV/c^{2} in the 3(π^{+}π^{-}) mass spectrum in J/ψ→γ3(π^{+}π^{-}) 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 +7.1 -2.6 MeV/c^2 and Gamma = 83 +- 14 +- 11 MeV. The product branching fraction is determined to be B(J/psi -> gammaX(1840)) x B(X(1840) -> 3(pi+pi-)) = (2.44+-0.36 +0.60 -0.74) 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.