2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/39919In this talk I summarize recently proposed mechanisms to understand pi pi scattering to 1 GeV in an effective chiral Lagrangian. The Lagrangian includes higher resonances in addition to pions consistently with the chiral symmetry. Iso-spin zero S-wave partial wave amplitude is reproduced up till about 1.2 GeV by including a pion self-interaction and resonant pole exchanges of rho, f0(980) and sigma derived from the effective chiral Lagrangian. The best fit shows that sigma has a mass of around 560 MeV and a width of about 370 MeV.6 pages, 2 figures, Talk given at YITP workshop ``Possible existence of the sigma-meson and its implications to hadron physics'' (June 12--14, 2000, Kyoto)High Energy Physics - Phenomenologytext