Studies on $\pi^+\pi^-$ phase motion in the $\Psi'\to J/\Psi \pi^+ \pi^-$ process

We propose a measurement of the elastic pi pi scattering phase-shift difference delta(0)(0)-delta(2)(0) through the Psi(')-> J/Psi pi(+)pi(-) process in the future high statistics BES-III experiment. The decay amplitude is constructed with seven Lorentz invariant form factors and is compared with their theoretical estimation. Based on a Monte Carlo study, it is found that the phase-shift difference can be indeed recovered and, hence, it is expected to be measured in the energy region between 350 and 550 MeV at future BES-III.