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Heavy neutrino searches from MeV to TeV

The Standard Model (SM) describes particle physics with great precision. However, it does not account for the generation of neutrino masses, whose nature we do not understand. Both a Dirac and a Majorana mass term could intervene, leading to the existence of heavy partners of the SM neutrinos, presumably more massive and nearly sterile. For suitable choices of parameters, heavy neutrinos can also provide dark matter candidates, and generate the observed baryon asymmetry of the universe. Heavy neutrinos can be searched for at beam dump facilities such as the proposed SHiP experiment if their mass is of the order of few GeV, or at high energy lepton colliders, such as the Future $e^+e^-$ Circular Collider, FCC-ee, presently under study at CERN, for higher masses. This contribution presents a review of the sensitivities for heavy neutrino searches at SHiP and FCC-ee.