2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/195957We illustrate the use of effective theory methods to describe resonant unstable particles. We outline the necessary ingredients to describe $W$-pair production close to threshold in $e^-e^+$ collisions.Invited talk at 2005 International Linear Collider Physics and Detector Workshop and Second ILC Accelerator Workshop, Snowmass, CO (Snowmass05), 3 pagesHigh Energy Physics - Phenomenologytext