Doubly-charged scalar bosons from the doublet

We consider the extended Higgs models, in which one of the isospin doublet scalar fields carries the hypercharge Y=3/2. Such a doublet field $\Phi_{3/2}$ is composed of a doubly charged scalar boson as well as a singly charged one. We first discuss a simple model with $\Phi_{3/2}$ (Model I), and study its collider phenomenology at the LHC. We then consider a new model for radiatively generating neutrino masses with a dark matter candidate (Model II), in which $\Phi_{3/2}$ and an extra Y=1/2 doublet as well as vector-like singlet fermions carry the odd quantum number for an unbroken discrete $Z_2$ symmetry. We also discuss the neutrino mass model (Model III), in which the exact $Z_2$ parity in Model II is softly broken. It is found that the doubly charged scalar bosons in these models show different phenomenological aspects from those which appear in models with a Y=2 isospin singlet field or a Y=1 triplet one. They could be clearly distinguished at the LHC.