  Small neutrino masses confirmed in the neutrino oscillation experiments
indicate the need for new physics beyond the standard model. Seesaw mechanism
is an interesting way to extend the standard model for explaining the neutrino
masses. In a low-scale type-I seesaw mechanism, the tiny masses of neutrinos
can be explained by heavy Majorana neutrino masses. Heavy Majorana neutrinos
can lead to lepton-number-violating processes and the induced CP violation can
contribute to the baryon asymmetry in the Universe. Heavy Majorana neutrinos
can also lead to lepton-number-conserving processes and in this paper, we study
the CP violation in lepton-number-conserving processes through heavy Majorana
neutrinos at future lepton colliders. New possible observations of CP violation
can also be connected to evidences of new physics beyond the standard model.
