arXiv subjects

Hiroshi Okada

Publications and source records attributed to Hiroshi Okada.

228 records · Page 13Linked to original sources

arXiv2010

Seesaw Mechanism Confronting PAMELA in S_4 Flavor Symmetric Extra U(1) Model

We study cosmic-ray anomaly observed by PAMELA based on E_6 inspired extra U(1) model with S_4 flavor symmetry. In our model, the lightest flavon has very long lifetime of O(10^18) second which is longer than the age of the universe, but not long enough to explain the PAMELA result O(10^26) sec. Such a situation could be avoidable by considering that the flavon is not the dominant component of dark matters. However non-thermalizing the flavon is needed to obtain proper relic density. This relates reheating temperature of the universe with seesaw mass scale. If we assume this flavon is a particle decaying into positron (or electron), the seesaw mass scale is constrained by reheating temperature. Thus we find an interesting result that the allowed region is around O(10^12) GeV, which is consistent with our original result.

hep-ph↗

arXiv2010

Cancellation Mechanism of FCNCs in S_4 x Z_2 Flavor Symmetric Extra U(1) Model

We propose a E_6 inspired supersymmetric model with a non-Abelian discrete flavor symmetry; SU(3)_c x SU(2)_W x U(1)_Y x U(1)_X x S_4 x Z_2. In our scenario, the additional abelian gauge symmetry; U(1)_X, not only solves the mu-problem in the minimal Supersymmetric Standard Model(MSSM), but also requires new exotic fields which play an important role in solving flavor puzzles. If our exotic quarks can be embedded into a S_4 triplet, which corresponds to the number of the generation, one finds that dangerous proton decay can be well-suppressed. Hence, it might be expected that the generation structure for lepton and quark in the SM(Standard Model) can be understood as a new system in order to stabilize the proton in a supersymemtric standard model (SUSY). Moreover, due to the nature of the discrete non-Abelian symmetry itself, Yukawa coupling constants of our model are drastically reduced. In our previous work, we actually have found much success. However, we also have to solve Higgs mediated FCNC at tree level, as is often the case with such extended Higgs models. In this paper, we propose a promising mechanism which could make cancellation between Higgs and SUSY contributions.

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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.

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arXiv2010

Non-Abelian Discrete Symmetries in Particle Physics

We review pedagogically non-Abelian discrete groups, which play an important role in the particle physics. We show group-theoretical aspects for many concrete groups, such as representations, their tensor products. We explain how to derive, conjugacy classes, characters, representations, and tensor products for these groups (with a finite number). We discussed them explicitly for $S_N$, $A_N$, $T'$, $D_N$, $Q_N$, $Σ(2N^2)$, $Δ(3N^2)$, $T_7$, $Σ(3N^3)$ and $Δ(6N^2)$, which have been applied for model building in the particle physics. We also present typical flavor models by using $A_4$, $S_4$, and $Δ(54)$ groups. Breaking patterns of discrete groups and decompositions of multiplets are important for applications of the non-Abelian discrete symmetry. We discuss these breaking patterns of the non-Abelian discrete group, which are a powerful tool for model buildings. We also review briefly about anomalies of non-Abelian discrete symmetries by using the path integral approach.

hep-th↗

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.

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arXiv2009

Delta(54) Flavor Model for Leptons and Sleptons

We have studied $Δ(54)\times Z_2$ flavor model for leptons and sleptons. The tri-bimaximal mixing can be reproduced for arbitrary neutrino masses if vacuum alignments of scalar fields are guaranteed. The deviation from the tri-bimaximal mixing of leptons is predicted. The predicted upper bound for $\sinθ_{13}$ is 0.06. The magnitude of $\sinθ_{23}$ could be deviated from the maximal mixing considerably, but $\sinθ_{12}$ is hardly deviated from the tri-maximal mixing. We have also studied SUSY breaking terms in the slepton sector. Three families of left-handed and right-handed slepton masses are degenerate. Even although flavor symmetry breaking effects are taken into account, our model leads to smaller values of flavor changing neutral currents than the present experimental bounds.

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arXiv2009

Lepton Flavor Model from Delta(54) Symmetry

We present the lepton flavor model with $Δ(54)$, which appears typically in heterotic string models on the $T^2/Z_3$ orbifold. Our model reproduces the tri-bimaximal mixing in the parameter region around degenerate neutrino masses or two massless neutrinos. We predict the deviation from the tri-bimaximal mixing by putting the experimental data of neutrino masses in the normal hierarchy of neutrino masses. The upper bound of $\sin^2θ_{13}$ is 0.01. There is the strong correlation between $θ_{23}$ and $θ_{13}$. Unless $θ_{23}$ is deviated from the maximal mixing considerably, $θ_{13}$ remains to be tiny.

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arXiv2008

Signatures for right-handed neutrinos at the Large Hadron Collider

We explore possible signatures for right-handed neutrinos in TeV scale B-L extension of the Standard Model (SM) at the Large Hadron Collider (LHC). The studied four lepton signal has a tiny SM background. We find the signal experimentally accessible at LHC for the considered parameter regions.

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arXiv2008

Model Bulding for the Neutrino Sector and Cold Dark Matter

It is now clear that the masses of the neutrino sector are much lighter than those of the other three sectors.There are many attempts to explain the neutrino masses radiatively by means of inert Higgses, which don't have vacuum expectation values. Then one can discuss cold dark matter candidates, because of no needing so heavy particles and having a $Z_2$ parity symmetry corresponding to the R-parity symmetry of the MSSM. The most famous work would be the Zee model. Recently a new type model along this line of thought was proposed by E. Ma. We introduce a flavor symmetry based on a dihedral group $D_6$ to constrain the Yukawa sector. For the neutrino sector, we find that the maximal mixing of atmospheric neutrinos is realized, it can also be shown that only an inverted mass spectrum, the value of $|V_{MNS_{13}}|$ is 0.0034 and so on. For the fermionic CDM candidates, we find that the mass of the CDM and the inert Higgs should be larger than about 230 and 300 GeV, respectively. If we restrict ourselves to a perturbative regime, they should be lighter than about 750 GeV.

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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.

hep-ph↗

arXiv2008

Cosmological Constraints for the Cold Dark Matter and Model Building based on the Flavor Symmetric Radiative Seesaw Model

It is now clear that the masses of the neutrino sector are much lighter than those of the other three sectors.There are many attempts to explain the neutrino masses radiatively by means of inert Higgses, which don't have vacuum expectation values. Then one can discuss cold dark matter candidates, because of no needing so heavy particles and having a $Z_2$ parity symmetry corresponding to the R-parity symmetry of the MSSM. The most famous work would be the Zee model. Recently a new type model along this line of thought was proposed by Mr. E. Ma. We paid attention to this idea. We introduce a flavor symmetry based on a dihedral group $D_6$ to constrain the Yukawa sector. For the neutrino sector, we find that the maximal mixing of atmospheric neutrinos is realized, it can also be shown that only an inverted mass spectrum, the value of $|V_{MNS_{13}}|$ is 0.0034 and so on. When one extends the Higgs sector, it leads to FCNCs mediated by Higgs fields generally. But in our model, the FCNCs are (of course) suppressed for the experiments sufficiently. For the fermionic CDM candidates, we find that the mass of the CDM and the inert Higgs should be larger than about 230 and 300 GeV, respectively. If we restrict ourselves to a perturbative regime, they should be lighter than about 750 GeV.

hep-ph↗

arXiv2004

Higgs Potential in Minimal S_3 Invariant Extension of the Standard Model

Minimal $S_{3}$ invariant Higgs potential with real soft $S_{3}$ breaking masses is investigated. It is required that without having a problem with triviality, all physical Higgs bosons, except one neutral one, become heavy ~O(10) TeV in order to sufficiently suppress flavor changing neutral currents. There exist three nonequivalent soft mass terms that can be characterized according to their discrete symmetries, and the one which breaks $S_{3}$ completely. The $S_{2}'$ invariant vacuum expectation values (VEVs) of the Higgs fields are the most economic VEVs in the sense that the freedom of VEVs can be completely absorbed into the Yukawa couplings so that it is possible to derive, without referring to the details of the VEVs, the most general form for the fermion mass matrices in minimal $S_{3}$ extension of the standard model. We find that except for the completely broken case of the soft terms, the $S_{2}'$ invariant VEVs are unique VEVs that satisfy the requirement of heavy Higgs bosons. It is found that they also correspond to a local minimum in the completely broken case.

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