Precision measurements, extra quark-lepton generations and 50 GeV neutrinos

The 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 $e^+ e^- \to N\bar{N}\gamma$. Existence of 50 GeV neutrinos makes standard model Higgs boson invisible.