2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/177440Applying the Amplitude Difference method to Fermilab experiment E791 D+ -> pi-pi+pi+ data, we measure the low mass pi+pi- phase motion. Our results suggest a significant phase variation, compatible with the existence of an isoscalar sigma(500) meson, as previously reported using an isobar model fit to the full Dalitz-plot density.12 pages, 8 figures Accepted for publication at PLBHigh Energy Physics - Experimenttext