Direct searches of extra Higgs boson at future colliders

We study direct searches of additional Higgs bosons in multi-top-quarks events at the LHC with the collision energy of 14 TeV as well as the International Linear Collider (ILC) with the collision energy of 1 TeV. As a benchmark model, we consider two Higgs doublet models with a softly-broken discrete $Z_2$ symmetry, where the $t\bar t$ decay mode of additional neutral Higgs bosons can be dominant if their masses are heavy enough. Thus, the multi-top-quarks events become an important probe of the extended Higgs sector at future colliders. We estimate the discovery reach at the LHC and the ILC, and find that the search at the ILC can survey the parameter regions where the LHC cannot cover.