Effect of the ηη channel and interference phenomena in the two-pion transitions of charmonia and bottomoniaYu.S. Surovtsev, P. Bydžovský, T. Gutsche, R. Kamiński, V.E. Lyubovitskij and M. NagyEPJ Web Conf., 138 (2017) 01025DOI: https://doi.org/10.1051/epjconf/201713801025