2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/188204We consider models where one Majorana neutrino is massless at tree level (like the see saw with two right-handed neutrinos), and compute the contribution to its mass m generated by two-loop quantum corrections. The result is m \sim 10^{-13}eV in the SM and m \sim 10^{-10} eV (tanBeta/10)^4 in the MSSM, compatible with the restricted range suggested by Affleck-Dine baryogenesis.6 pages. v2: final version to appear on PLB; important reference addedHigh Energy Physics - Phenomenologytext