Inert Extension of the Zee-Babu Model

We propose a two-loop induced Zee-Babu type neutrino mass model at the TeV scale. Although there is no dark matter candidate in the original Zee-Babu model, that is contained in our model by introducing an unbroken discrete $Z_2$ symmetry. The discrepancy between the experimental value of the muon anomalous magnetic moment (muon $g-2$) and its prediction in the standard model can be explained by contributions from additional vector-like charged-leptons which are necessary to give non-zero neutrino masses. The mass of vector-like leptons to be slightly above 300 GeV is favored and allowed from the muon $g-2$ and the current LHC data. We find that from the structure of neutrino mass matrix, doubly-charged scalar bosons in our model can mainly decay into the same-sign and same-flavour dilepton plus missing transverse momentum. By measuring an excess of these events at the LHC, our model can be distinguished from the other models including doubly-charged scalar bosons.

Radiative Generation of Lepton Masses with the $U(1)$ Gauge Symmetry

We revisit our previous model proposed in Ref. cite{Okada:2013iba}, in which lepton masses except the tauon mass are generated at the one-loop level in a TeV scale physics. Although in the previous work, rather large Yukawa couplings constants; i.e., greater than about 3, are required to reproduce the muon mass, we do not need to introduce such a large but ${cal O}$(1) couplings. In our model, masses for neutrinos (charged-leptons) are generated by a dimension five effective operator with two isospin triplet (singlet and doublet) scalar fields. Thus, the mass hierarchy between neutrinos and charged-leptons can be naturally described by the difference in the number of vacuum expectation values (VEVs) of the triplet fields which must be much smaller than the VEV of the doublet field due to the constraint from the electroweak rho parameter. Furthermore, the discrepancy in the measured muon anomalous magnetic moment ($g-2$) from the prediction in the standard model are explained by one-loop contributions from vector-like extra charged-leptons which are necessary to obtain the radiative generation of the lepton masses. We study the decay property of the extra leptons by taking into account the masses of muon, neutrinos, muon $g-2$ and dark matter physics. We find that the extra leptons can mainly decay into the mono-muon, dark matter with or without $Z$ bosons in the favored parameter regions.

$T_7$ Flavor Model in Three Loop Seesaw and Higgs Phenomenology

We propose a new type of radiative seesaw model in which observed neutrino masses are generated through a three-loop level diagram in combination with tree-level type-II seesaw mechanism in a renormalizable theory. We introduce a Non-abelian flavor symmetry $T_7$ in order to constrain the form of Yukawa interactions and Higgs potential. Although several models based on a Non-abelian flavor symmetry predict the universal coupling constants among the standard model like Higgs boson and charged leptons, which is disfavored by the current LHC data, our model can avoid such a situation. We show a benchmark parameter set that is consistent with the current experimental data, and we discuss multi-muon events as a key collider signature to probe our model.

Two Loop Radiative Seesaw Model with Inert Triplet Scalar Field

We propose a radiative seesaw model with an inert triplet scalar field in which Majorana neutrino masses are generated at the two loop level. There are fermionic or bosonic dark matter candidates in the model. We find that each candidate can satisfy the WMAP data when its mass is taken to be around the half of the mass of the standard model like Higgs boson. We also discuss phenomenology of the inert triplet scalar bosons, especially focusing on the doubly-charged scalar bosons at Large Hadron Collider in parameter regions constrained by the electroweak precision data and WMAP data. We study how we can distinguish our model from the minimal Higgs triplet model.