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 $\pi \pi$, $K\bar{K}$ 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.