500 1000 1500 s [GeV] 10 1 10 2 10 3 10 4 10 5 Cross section [fb] AKS e e S S Zee-Babu e e SM e e W W e e FIG. 5: The cross sections of like-sign charged Higgs pair productions in the Zee-Babu model ( ω $^{−}$ω$^{−}$) and in the AKS model ( S $^{−}$S$^{−}$) are shown as a function of the collision energy √ s . The parameters in the Zee-Babu and the AKS model are taken as in Eq. (11) and Eq. (13), respectively. than 10 − 4 fb because the coupling constants ˆ h α i are very small in the parameters in Eq. (12). Allowing some fine tuning, ˆ h α i may be at most 0.01 for heavier N α $_{R}$. In any case, the cross section of e $^{−}$e− → ξ $^{−}$ξ− is smaller than 10 − 3 fb. Hence, most of the successful scenarios in the Ma model the process e $^{−}$e− → ξ $^{−}$ξ− is difficult to be seen. In the AKS model, the cross section of e $^{−}$e− → S $^{−}$S − is large, and its value amounts to about 15 pb at √$_{s}$ = 1 TeV in the scenario given in Eq. (13). Above the threshold, the magnitude of the cross sections are not sensitive to √$_{s}$$_{, so that even if}$ m$_{S}$ ± would be at the TeV scale, we might be able to test it at future multi-TeV linear colliders, such as the Compact Linear Collider [35]. Because B ( S ± → η $^{0}$H $^{±}$) ≃ B ( H ± → τ $^{±}$ν ) ≃ 100 %, the signal should be τ $^{−}$τ $^{−}$ννηη with almost the same rate as long as m$_{S}$ ± < m$_{N}$ α $_{R}$. The background mainly comes from W $^{−}$W $^{−}$ν$_{e}$ ν$_{e}$ , and the cross section is about 2.3 fb (22 fb) for √$_{s}$ = 500 GeV (1 TeV). The branching ratio for the leptonic decay of W bosons is 30%, so that the rate of the final state ℓℓ $^{′}$νννν is at most 2 fb or less. Therefore, the signal in the AKS model and in the Zee-Babu model can be seen. Apart from the TeV-scale radiative seesaw models, there are many models with lepton number violating interactions or right-handed Majorana neutrinos. Atwood et al. have discussed the signature of heavy Majorana neutrinos in the model without Z$_{2}$ symmetry via 16