Seesaw mechanism at electron-electron colliders

We consider the Standard Model with right-handed neutrinos to explain the masses of active neutrinos by the seesaw mechanism. Since active neutrinos as well as heavy neutral leptons are Majorana fermions in this case, the lepton number violating process can be induced. We discuss the inverse neutrinoless double beta decay $e^- e^- \to W^- W^-$ in the framework of the seesaw mechanism and its detectability at future colliders. It is shown that the cross section can be 17 fb for $\sqrt{s}=3$ TeV even with the stringent constraint from the neutrinoless double beta decays if three (or more) right-handed neutrinos exist. In such a case, the future $e^- e^-$ colliders can test lepton number violation mediated by a right-handed neutrino lighter than about 10 TeV.