2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/182793The pion mass difference generates a pronounced cusp in K --> 3 pi decays. As has recently been pointed out by Cabibbo and Isidori, an accurate measurement of the cusp may allow one to pin down the S-wave pi pi scattering lengths to high precision. Here, we present and illustrate an effective field theory framework that allows one to determine the structure of this cusp in a straightforward manner. The strictures imposed by analyticity and unitarity are respected automatically.14 pages, 3 figures, uses Elsevier styleHigh Energy Physics - Phenomenologytext