Doubly-charged Higgs bosons in the diboson decay scenario at the ILC

The Higgs Triplet Model (HTM) is one of important examples for extended Higgs sectors, because tiny neutrino masses can be simply explained. Unlike the canonical type-I seesaw model, a scale of new particles can be taken as $\mathcal{O}(100)$ GeV keeping an enough amount of production cross section for direct searches at collider experiments. In the HTM, there appear doubly-charged Higgs bosons $H^{\pm\pm}$, and detection of them is a key to probe the model. The decay property of $H^{\pm\pm}$ depends on the magnitude of the vacuum expectation value of the triplet field $v_\Delta$. When $v_\Delta$ is smaller than about 1 MeV, $H^{\pm\pm}$ can mainly decay into the same-sign dilepton, and the lower mass limit for $H^{\pm\pm}$ had been taken to be about 400 GeV at the LHC. On the other hand, if $v_\Delta$ is larger than about 1 MeV, $H^{\pm\pm}$ can mainly decay into the same-sign diboson. In this case, the mass bound cannot be applied, so that the scenario based on light $H^{\pm\pm}$ is still possible. In this talk, we discuss the phenomenology of the same-sign diboson decay scenario of $H^{\pm\pm}$. First, we review the mass bound from the current collider experiments given in Ref. \cite{KYY}. We then discuss the strategy for detection of $H^{\pm\pm}$ at the ILC.