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

Abstract

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.