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Yuji Kajiyama

Publications and source records attributed to Yuji Kajiyama.

At least 19 recordsLinked to original sources
arXiv2014

Electron/Muon Specific Two Higgs Doublet Model

We discuss two Higgs doublet models with a softly-broken discrete $\mathbb{S}_3$ symmery, where the mass matrix for charged-leptons is predicted as the diagonal form in the weak eigenbasis of lepton fields. Similar to an introduction of $\mathbb{Z}_2$ symmetry, the tree level flavor changing neutral current can be forbidden by imposing the $\mathbb{S}_3$ symmetry to the model. Under the $\mathbb{S}_3$ symmetry, there are four types of Yukawa interactions depending on the $\mathbb{S}_3$ charge assignment to right-handed fermions. We find that extra Higgs bosons can be muon and electron specific in one of four types of the Yukawa interaction. This property does not appear in any other two Higgs doublet models with a softly-broken ${\mathbb Z}_2$ symmetry. We discuss the phenomenology of the muon and electron specific Higgs bosons at the Large Hadron Collider; namely we evaluate allowed parameter regions from the current Higgs boson search data and discovery potential of such a Higgs boson at the 14 TeV run.

hep-ph↗
arXiv2013

New Interpretation of the Recent Result of AMS-02 and Multi-component Decaying Dark Matters with non-Abelian Discrete Flavor Symmetry

Recently the AMS-02 experiment has released the data of positron fraction with much small statistical error. Because of the small error, it is no longer easy to fit the data with a single dark matter for a fixed diffusion model and dark matter profile. In this paper, we propose a new interpretation of the data that it originates from decay of two dark matter. This interpretation gives a rough threshold of the lighter DM component. When DM decays into leptons, the positron fraction in the cosmic ray depends on the flavor of the final states, and this is fixed by imposing non-Abelian discrete symmetry in our model. By assuming two gauge-singlet fermionic decaying DM particles, we show that a model with non-Abelian discrete flavor symmetry, e.g. $T_{13}$, can give a much better fitting to the AMS-02 data compared with single dark matter scenario. Few dimension six operators of universal leptonic decay of DM particles are allowed in our model since its decay operators are constrained by the $T_{13}$ symmetry. We also show that the lepton masses and mixings are consistent with current experimental data, due to the flavor symmetry.

hep-ph↗
arXiv2013

One-parameter Neutrino Mass Matrix and Symmetry Realization

We investigate the Majorana neutrino mass matrix $M_ν$ with one parameter in the context of two texture zeros and its symmetry realization by non-Abelian discrete symmetry. From numerical calculation, we confirm that the textures $(M_ν)_{11,12}=0$ and $(M_ν)_{11,13}=0$ are consistent with the current experimental constraints, and show the correlations between non-zero elements of $M_ν$. The ratios of non-zero elements of $M_ν$ are constrain in small regions, and we find simple examples of $M_ν$ with one real mass parameter. We also discuss symmetry realization of the mass matrix by the type-II seesaw mechanism based on the binary icosahedral symmetry $A_5'$ .

hep-ph↗
arXiv2013

$T_7$ 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.

hep-ph↗
arXiv2013

Multicomponent dark matter particles in a two-loop neutrino model

We construct a loop induced seesaw model in a TeV scale theory with gauged U(1)_{B-L} symmetry. Light neutrino masses are generated at two-loop level and right-handed neutrinos also obtain their masses by one-loop effect. Multi-component Dark Matters (DMs) are included in our model due to the remnant discrete symmetry after the B-L symmetry breaking and the Z_2 parity which is originally imposed to the model. We investigate the multi-component DM properties, in which we have two fermionic DMs with different mass scales, O(10) GeV and O(100-1000) GeV. The former mass corresponds to the lightest right-handed neutrino mass induced by the loop effect, although the latter one to the SM gauge singlet fermion. We show each of the DM annihilation processes and compare to the the observation of relic abundance, together with the constraints of Lepton Flavor Violation (LFV) and active neutrino masses. Moreover we show that our model has some parameter region allowed by the direct detection result reported by XENON100, and it is possible to verify the model by the future XENON experiment.

hep-ph↗
arXiv2013

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.

hep-ph↗
arXiv2013

Light Dark Matter Candidate in B-L Gauged Radiative Inverse Seesaw

We study a radiative inverse seesaw model with local B-L symmetry, in which we extend the neutrino mass structure that is generated through a kind of inverse seesaw framework to the more generic one than our previous work. We focus on a real part of bosonic particle as a dark matter and investigate the features in O(1-80) GeV mass range, reported by the experiments such as CoGeNT and XENON (2012).

hep-ph↗
arXiv2011

Symmetryless Dark Matter

It is appealing to stabilize dark matter by the same discrete symmetry that is used to explain the structure of quark and lepton mass matrices. However, to generate the observed fermion mixing patterns, any flavor symmetry must necessarily be broken, rendering dark matter unstable. We study singlet, doublet and triplet SU(2) multiplets of both scalar and fermion dark matter candidates and enumerate the conditions under which no d < 6 dark matter decay operators are generated even in the case if the flavor symmetry is broken to nothing. We show that the VEVs of flavon scalars transforming as higher multiplets (e.g. triplets) of the flavor group must be at the electroweak scale. The most economical way for that is to use SM Higgs boson(s) as flavons. Such models can be tested by the LHC experiments. This scenario requires the existence of additional Froggatt-Nielsen scalars that generate hierarchies in Yukawa couplings. We study the conditions under which large and small flavor breaking parameters can coexist without destabilizing the dark matter.

hep-ph↗
arXiv2011

A light Scalar Dark Matter for CoGeNT and DAMA in D_6 Flavor Symmetric Model

We try to interpret a very light dark matter with mass of 5~10 GeV which is in favor of the recent experiments reported by CoGeNT and DAMA, in a non-supersymmetric extension of radiative seesaw model with a family symmetry D_6 x Z_2 x Z_2. We show that a D_6 singlet real scalar field can be a promising dark matter candidate, and it gives the elastic cross section σ\simeq 7x10^{-41} cm^2 which is required by these experiments. Our dark matter interacts with a D_6 singlet scalar Higgs boson, which couples only to quark sector. The dark matter-nucleon cross section and new decay mode h->DM DM can be large if the standard model Higgs boson h is light. The Higgs phenomenology is also discussed.

hep-ph↗
arXiv2011

Direct and Indirect Detection of Dark Matter in D6 Flavor Symmetric Model

We study a fermionic dark matter in a non-supersymmetric extension of the standard model with a family symmetry based on D6xZ2xZ2. In our model, the final state of the dark matter annihilation is determined to be e+ e- by the flavor symmetry, which is consistent with the PAMELA result. At first, we show that our dark matter mass should be within the range of 230 GeV - 750 GeV in the WMAP analysis combined with mu to e gamma constraint. Moreover we simultaneously explain the experiments of direct and indirect detection, by simply adding a gauge and D6 singlet real scalar field. In the direct detection experiments, we show that the lighter dark matter mass ~ 230 GeV and the lighter standard model Higgs boson ~ 115 GeV is in favor of the observed bounds reported by CDMS II and XENON100. In the indirect detection experiments, we explain the positron excess reported by PAMELA through the Breit-Wigner enhancement mechanism. We also show that our model is consistent with no antiproton excess suggested by PAMELA.

hep-ph↗
arXiv2011

Like-sign dimuon asymmetry of B0 meson and LFV in SU(5) SUSY GUT with S4 flavor symmetry

The like-sign dimuon charge asymmetry of the $B$ meson, which was reported in the D$Ø$ Collaboration, is studied in the SU(5) SUSY GUT model with $S_4$ flavor symmetry. Additional CP violating effects from the squark sector are discussed in $B_s-\bar B_s$ mixing process. The predicted like-sign charge asymmetry is in the 2$σ$ range of the combined result of D$Ø$ and CDF measurements. Since the SUSY contributions in the quark sector affect to the lepton sector because of the SU(5) GUT relation, two predictions are given in the leptonic processes: (i) both ${\rm BR}(μ\to e γ)$ and the electron EDM are close to the present upper bound, (ii) the decay ratios of $τ$ decays, $τ\to μγ$ and $τ\to e γ$, are related to each other via the Cabibbo angle $λ_c$: ${\rm BR}(τ\to eγ)/{\rm BR}(τ\to μγ)\sime λ_c^2$. These are testable at future experiments.

hep-ph↗
arXiv2011

T(13) Flavor Symmetry and Decaying Dark Matter

We study a new flavor symmetric model with non-Abelian discrete symmetry T_{13}. The T_{13} group is isomorphic to Z_{13} \rtimes Z_3, and it is the minimal group having two complex triplets as the irreducible representations. We show that the T_{13} symmetry can derive lepton masses and mixings consistently. Moreover, if we assume a gauge-singlet fermionic decaying dark matter, its decay operators are also constrained by the T_{13} symmetry so that only dimension six operators of leptonic decay are allowed. We find that the cosmic-ray anomalies reported by PAMELA and Fermi-LAT are explained by decaying dark matter controlled by the T_{13} flavor symmetry.

hep-ph↗
arXiv2010

Universally Leptophilic Dark Matter From Non-Abelian Discrete Symmetry

The positron anomaly recently reported by the cosmic-ray measurements can be explained by the decaying dark matter scenario, where it decays mainly into leptons with the lifetime of O(10^26) second. When the dark matter is a fermionic particle, the lifetime of this order is known to be obtained by a dimension 6 operator suppressed by the unification scale 10^16 GeV, while such decay operators do not necessarily involve only leptons. In addition, the scenario would be spoiled if there exist lower-dimensional operators inducing the dark matter decay. We show in this letter that a single non-Abelian discrete symmetry such as A_4 is possible to prohibit all such harmful (non-leptonically coupled and lower-dimensional) operators. Moreover, the dark matter decays into charged leptons in a flavor-blind fashion due to the non-Abelian flavor symmetry, which results in perfect agreements not only with the PAMELA data but also with the latest Fermi-LAT data reported very recently. We also discuss some relevance between the discrete symmetry and neutrino physics.

hep-ph↗
arXiv2010

Gauged B-L Leptogenesis

We propose a new leptogenesis scenario in a gauged $B-L$ model with supersymmetry at the TeV energy scale. Instead of relying on the very small Yukawa couplings of the singlet neutrinos $N^c$ to generate the observed baryon asymmetry of the Universe, which requires a very large resonance enhancement, their $B-L$ gauge interactions are invoked. Successful leptogenesis is then possible if a particular scalar bilinear $\tilde N^c \tilde N^c$ term is disallowed.

hep-ph↗
arXiv2009

Electron EDM and soft leptogenesis in supersymmetric B-L extension of the standard model

We analyze the connection between electric dipole moment of the electron and the soft leptogenesis in supersymmetric $B-L$ extension of the standard model. In this model, the $B-L$ symmetry is radiatively broken at TeV scale. Therefore, it is a natural framework for low scale seesaw mechanism and also for implementing the soft leptogenesis. We show that the phases of trilinear soft SUSY breaking couplings $A$, which are relevant for the lepton asymmetry, are not constrained by the present experimental bounds on electric dipole moment. As in the MSSM extended with right-handed neutrinos, successful leptogenesis requires small bilinear coupling $B$, which is now given by $A_N$ and $B-L$ breaking VEVs. SUSY $B-L$ model with non-universal $A$-terms such that $A_N=0$ while $A_ν\neq0$ is a promising scenario for soft leptogenesis. The proposed EDM experiments will test this scenario in the future.

hep-ph↗
arXiv2008

Muon Anomalous Magnetic Moment and Lepton Flavor Violating Tau Decay in Unparticle Physics

We study effects of unparticle physics on muon g-2 and LFV tau decay processes. LFV interactions between the Standard Model sector and unparticles can explain the difference of experimental value of muon g-2 from the Standard Model prediction. While the same couplings generate LFV tau decay, we found that LFV coupling can be of O(0.1 ... 1) without conflict with experimental bounds of LFV tau decay if the scaling dimension of unparticle operator d_{U} > 1.6.

hep-ph↗
arXiv2008

D6 Family Symmetry and Cold Dark Matter at LHC

We consider a non-supersymmetric extension of the standard model with a family symmetry based on D6 Z2 Z2, where one of Z2's is exactly conserved. This Z2 forbids the tree-level neutrino masses and simultaneously ensures the stability of cold dark matter candidates. From the assumption that cold dark matter is fermionic we can single out the D6 singlet right-handed neutrino as the best cold dark mater candidate. We find that an inert charged Higgs with a mass between 300 and 750 GeV decays mostly into an electron (or a positron) with a large missing energy, where the missing energy is carried away by the cold dark matter candidate. This will be a clean signal at LHC.

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