2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/42352We study neutrino oscillations in vacuum and in matter using field theoretical methods and wave-packets. In particular, we calculate the neutrino propagator in the presence of matter with constant density for the case of two flavors, obtaining the resonance formula. In the extreme relativistic limit, the result of the usual quantum mechanical treatment is recovered with interesting, but small modifications.17 pages, RevTeXHigh Energy Physics - Phenomenologytext