charged Higgs pair production at e $^{+}$e − and e $^{−}$e− collisions [36]. 10 In supersymmetric models, Majorana particles also appear, and their effects also give similar t -channel contributions to the above models in the slepton pair production through the gauge couplings, e $^{−}$e− → ˜ e $^{−}$˜ e $^{−}$, whose cross section is of O (100) fb. The final state would be e $^{−}$e$^{−}$χ $^{0}$χ0 for example. 

The e $^{−}$e− collision experiment is useful to test the Majorana nature of radiative seesaw models such as the Zee-Babu model and the AKS model via like-sign pair production of charged scalar bosons. The cross section can be significant and hierarchical among these models. The signal can be observed as a model dependent final state, by which we can discriminate the models. Although in this Letter we did not explicitly discuss the KNT model that also contains TeV-scale right-handed neutrinos N α $_{R}$, we found that the cross section of e $^{−}$e− → S − 2 S − 2 ( S − 2 is the Z$_{2}$ -odd isosinglet charged scalar boson) is very small because the coupling constants for e$_{R}$N α $_{R}$S − 2 ( α = 1 , 2) are tiny [12]. 

## V. CONCLUSION 

We have discussed general features of TeV-scale radiative seesaw models. They are char- acterized by an extended scalar (Higgs) sector and the Majorana nature. We have mainly discussed the concrete models with neutrino mass generation at one-loop (the Ma model), two-loop (the Zee-Babu model), and at three-loop (the model in Ref. [6]). Various phe- nomenological aspects of these models have been discussed especially in experiments at the LHC and at the ILC. We have found that, while the extended Higgs sector can be explored at the LHC, the Majorana nature of the models can directly be tested at the ILC via the pair production of the charged scalar bosons at the electron-positron and electron-electron collision experiments. The detailed realistic simulation has to be done elsewhere. 

## Acknowledgments 

We like to thank Ernest Ma for useful discussions. The work of SK was supported, in part, by Grant-in-Aid for scientific research (C), Japan Society for the Promotion of Science 

