Extracting ππ S-wave scattering lengths from cusp effect in heavy quarkonium dipion transitions Xiao-Hai Liu, Feng-Kun Guo and Evgeny Epelbaum Eur. Phys. J. C, 73 1 (2013) 2284 Published online: 26 January 2013 DOI: 10.1140/epjc/s10052-013-2284-2