2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/204657The pion mass difference generates a pronounced cusp in K --> 3 pi decays, the strength of which is related to the pi pi S-wave scattering lengths. We apply an effective field theory framework developed earlier to evaluate the amplitudes for K_L --> 3 pi decays in a systematic manner, where the strictures imposed by analyticity and unitarity are respected automatically. The amplitudes for the decay eta --> 3 pi are also given.15 pages, 3 figures, uses Elsevier styleHigh Energy Physics - Phenomenologytext