Studies on $\pi^+\pi^-$ phase motion in the $\Psi'\to J/\Psi \pi^+   \pi^-$ process
  We propose a measurement on the elastic $\pi\pi$ scattering phase shift difference $\delta^0_0-\delta^2_0$ through $\Psi'\to J/\Psi \pi^+ \pi^-$ process in future high statistics BES-III experiment. The decay amplitude is constructed with seven Lorentz invariant form-factors and is compared with their theoretical estimation. It is found that the phase shift difference can be obtained, based on a Monte Carlo study and it is expected the phase shift in the energy region between 350 MeV to 550 MeV can be measured at future BES-III. 
Z.X. Zhang, J.J. Sanz-Cillero, X.Y. Shen, N. Wu, L.Y. Xiao, H.Q. Zheng