Effect of the $\eta\eta$ channel and interference phenomena in the   two-pion transitions of charmonia and bottomonia
  The basic shape of di-pion mass spectra in the two-pion transitions of both charmonia and bottomonia states is explained by an unified mechanism based on contributions of the $\pi\pi$, $K\overline{K}$ and $\eta\eta$ coupled channels including their interference. The role of the individual $f_0$ resonances in shaping the di-pion mass distributions in the charmonia and bottomonia decays is considered. 
Yu.S. Surovtsev, P. Byd\v{z}ovsk\'y, T. Gutsche, R. Kami\'nski, V.E.   Lyubovitskij, M. Nagy
hep-ph