Heavy Majorana neutrinos in electron-positron and electron-proton collisions

Heavy Majorana neutrinos can be produced in charged and neutral current processes through their mixing with light neutrinos. We calculate production cross sections and final-state lepton spectra for e+e− annihilation and ep scattering, and compare the discovery potentials of present and future e+e− and ep colliders. At HERA heavy neutrinos with masses up to 160 GeV can be discovered, at LEP200 masses up to 190 GeV can be reached. The 500 GeV e+e− collider will probe for the first time heavy neutrino couplings to leptons of the second and third family. The most direct evidence for lepton number violation can be provided by ep colliders due to the nonzero lepton number of the initial state.