Measurement of the Matrix Element for the Decay $\eta^{\prime} \to \eta \pi^+\pi^-$

Understand

The Dalitz plot of $\eta^{\prime} \to \eta \pi^+ \pi^-$ decay is studied using $(225.2\pm2.8)\times 10^6$ $\jpsi$ events collected with the BESIII detector at the BEPCII $e^+e^-$ collider.

• With the largest sample of $\eta^{\prime}$ decays to date, the parameters of the Dalitz plot are determined in a generalized and a linear representation.
• Also the branching fraction of $\jpsi \to \gamma \eta^{\prime}$ is determined to be $(4.84\pm0.03\pm0.24)\times 10^{-3}$, where the first error is statistical and the second systematic.