Observation of a structure at 1.84 GeV/c${}^2$ in the $3({\pi }^{+}{\pi }^{-})$ mass spectrum in $J/\psi \to \gamma 3({\pi }^{+}{\pi }^{-})$ decays

Abstract

With a sample of 225.3 million $J/\psi$ events taken with the BESIII detector, the decay $J/\psi \to \gamma 3({\pi }^{+}{\pi }^{-})$ 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 $M=1842.2\pm 4.2_{-2.6}^{+7.1}$ MeV/c${}^2$ and $\Gamma =83\pm 14\pm 11$ MeV. The product branching fraction is determined to be $B(J/\psi \to \gamma X(1840))\times B(X(1840)\to 3({\pi }^{+}{\pi }^{-}))=(2.44\pm 0.36_{-0.74}^{+0.60})\times 10^{-5}$. No ${\eta }^{\prime }$ signals are observed in the $3({\pi }^{+}{\pi }^{-})$ invariant mass spectrum, and the upper limit of the branching fraction for the decay ${\eta }^{\prime }\to 3({\pi }^{+}{\pi }^{-})$ is set to be $3.1\times 10^{-5}$ at a 90% confidence level.

6 pages, 3 figures, minor changes, to be appeared in journal