Observation of a structure at 1.84 GeV=c² in the 3(π⁺ π^-) mass spectrum in J/φ → γ3(π⁺ π^-) decays

With a sample of 225.3 × 10⁶ J/φ events taken with the BESIII detector, the decay J/φ → γ³ (π⁺ π^-) is analyzed. A structure at 1.84 GeV/c² is observed in the 3(π⁺ π^-) invariant mass spectrum with a statistical significance of 7.6σ. The mass and width are measured to be M = 1842.2 ± 4.2_-2.6 ^+7.1 MeV/c² and Γ = 83 ± 14 ± 11 MeV. The product branching fraction is determined to be B (J/φ → γX(1840)) × B(X(1840) → 3(π⁺ π^-)) = (2.44 ± 0.36_-0.74 ^+0.60) × 10^-5. No η′ signals are observed in the 3(π⁺ π^-) invariant mass spectrum, and the upper limit of the branching fraction for the decay η′ → 3(π⁺ π^-) is set to be 3.1 × 10^-5 at a 90% confidence level.