Precision Study of ${\eta }^{\mathrm{\prime }}\to \gamma {\pi }^{+}{\pi }^{-}$ Decay Dynamics

Using a low background data sample of $9.7\times {10}^5$ $J\psi \to \gamma {\eta }^{\mathrm{\prime }}$, ${\eta }^{\mathrm{\prime }}\to \gamma {\pi }^{+}{\pi }^{-}$ events, which are 2 orders of magnitude larger than those from the previous experiments, recorded with the BESIII detector at BEPCII, the decay dynamics of ${\eta }^{\mathrm{\prime }}\to \gamma {\pi }^{+}{\pi }^{-}$ are studied with both model-dependent and model-independent approaches. The contributions of $\omega$ and the $\rho (770)-\omega$ interference are observed for the first time in the decays ${\eta }^{\mathrm{\prime }}\to \gamma {\pi }^{+}{\pi }^{-}$ in both approaches. Additionally, a contribution from the box anomaly or the $\rho (1450)$ resonance is required in the model-dependent approach, while the process specific part of the decay amplitude is determined in the model-independent approach.