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

TL;DR

This paper reports the observation of a new structure at 1.84 GeV/c² in the six-pion mass spectrum from J/ψ decays, with detailed measurements of its properties and implications for meson spectroscopy.

Contribution

The study provides the first observation of a structure at 1.84 GeV/c² in J/ψ decays with precise mass, width, and branching fraction measurements, enhancing understanding of light meson states.

Findings

Structure observed at 1.84 GeV/c² with 7.6σ significance.

Measured mass and width: 1842.2 MeV/c² and 83 MeV.

No significant η′ signal in the six-pion spectrum.

Abstract

With a sample of 225.3 million $J/\psi$ events taken with the BESIII detector, the decay $J/\psi\rightarrow \gamma 3(\pi^+\pi^-)$ 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 $M=1842.2\pm 4.2^{+7.1}_{-2.6}$ MeV/c$^2$ and $\Gamma=83\pm 14 \pm 11$ MeV. The product branching fraction is determined to be $B(J/\psi\rightarrow\gamma X(1840))\times B(X(1840)\rightarrow 3(\pi^+\pi^-))=(2.44\pm0.36^{+0.60}_{-0.74})\times 10^{-5}$. 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 $\eta^\prime\rightarrow 3(\pi^+\pi^-)$ is set to be $3.1\times10^{-5}$ at a 90% confidence level.