Possible enhancement of the $e^+ e^- \to H^\pm W^\mp$ cross section in the two-Higgs-doublet model

The production process of the charged Higgs-boson associated with a $W$ boson is discussed at electron-positron colliders in the general two-Higgs-doublet model (2HDM) as well as the minimal supersymmetric standard model (MSSM). This process is induced at one-loop level in these models. We investigate possible enhancement of the cross section due to the quark and Higgs one-loop effects under available experimental constraints and requirement from validity of perturbation theory. In the MSSM, quadratic mass effects of the top quark enlarge the cross section, while the Higgs-loop effects are very small because of the decoupling theorem. In the non-SUSY 2HDM, in addition to the large top-quark mass effects for small $\tan \beta$ region, the cross section can be enhanced by the Higgs non-decoupling effects in the intermediate $\tan \beta$ region. These effects are rapidly reduced for larger $\tan \beta$ region by the constraint from validity of perturbation theory.