The interference effects of multi-channel pion-pion scattering   contributions to the final states of $\Psi$- and $\Upsilon$-meson family   decays
  It is shown that the basic shape of dipion mass distributions in the two-pion transitions of both charmonia and bottomonia states are explained by an unified mechanism based on the contribution of the $\pi\pi$, $K\overline{K}$ and $\eta\eta$ coupled channels including their interference. 
Yu.S. Surovtsev, P. Byd\v{z}ovsk\'y, T. Gutsche, R. Kami\'nski, V.E.   Lyubovitskij, and M. Nagy
hep-ph