2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/126638In this talk we discuss an elementary derivation of the semi-classical spectrum of neutral particles in two field theories with kink excitations. We also show that, in the non-integrable cases, each vacuum state cannot generically support more than two stable particles, since all other neutral exitations are resonances, which will eventually decay.17 pages, 8 figures, to appear in the proceedings of the XVth International Congress on Mathematical Physics, Rio de Janeiro, 6-11 August 2006High Energy Physics - Theorytext