Observation of the $X(1840)$ at BESIII
Observation of the $X(1840)$ in the $3(\pi^+\pi^-)$ invariant mass in $J/\psi\rightarrow\gamma 3(\pi^+\pi^-)$ at BESIII is reviewed. With a sample of $225.3\times10^6$ $J/\psi$ events collected with the BESIII detector at BEPCII, the $X(1840)$ is observed with a statistical significance of $7.6\sigma$. The mass, width and product branching fraction of the $X(1840)$ are determined. The decay $\eta^\prime\rightarrow 3(\pi^+\pi^-)$ is searched for, and the upper limit of the branching fraction is set at the 90% confidence level.