Probing the heavy neutrino hypothesis

There is a strong experimental program searching for massive sterile neutrinos. Here we focus on the heavy regime above the GeV scale, where their existence can be probed in either high-energy colliders or in high-precision facilities. We first review the current experimental status at colliders, showing that the LHC already improves LEP results for mixings to electrons and muons, while mixings to taus remain challenging at a hadron collider. We also discuss the importance of exploring both lepton number violating and conserving signals, as well as different flavor channels. Finally, we present the latest global analysis of electroweak precision and flavor observables, showing that the intensity frontier provides the strongest constraints for heavy neutrino masses above the electroweak scale.