2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/40723The existence of extra chiral generations with all fermions heavier than $m_Z$ is strongly disfavored by the precision electroweak data. However the data still allow a few extra generations if the neutral leptons have masses close to 50 GeV. Such heavy neutrino can be searched in the reaction $e^+ e^- \to N\bar{N}γ$. Existence of 50 GeV neutrinos makes standard model Higgs boson invisible.11 pages, 4 figures. Talk at Les Rencontres de Physique de la Vallee d'Aoste, 2001High Energy Physics - Phenomenologytext