arXiv subjects

Midori Obara

Publications and source records attributed to Midori Obara.

11 recordsLinked to original sources

arXiv2007

The Possible Textures in the Seesaw Realization of the Strong Scaling Ansatz and the Implications for Thermal Leptogenesis

We classify the textures of the Dirac and the right-handed Majorana neutrino mass matrices, $M_D$ and $M_R$, which can satisfy the so-called ``Strong Scaling Ansatz'' (SSA) within the framework of the seesaw mechanism $M_ν=-M_D^T M_R^{-1} M_D$. We assume that the Dirac neutrino mass matrix has some texture zeros and examine which elements should be zero in order to satisfy the SSA, by taking into account all possible textures for $M_R$. We find that the resulting Dirac neutrino mass matrices have rank 2 as well as the rank of the effective neutrino mass matrix $M_ν$, or rank 1, depending only on the textures of $M_R^{-1}$. We also consider the three cases of the breaking of the SSA by introducing a complex breaking parameter in $M_ν$ and show that it can generate the CP violation in the lepton sector as well as non-zero $m_3$ and $U_{e3}$. We furthermore discuss the implications of the thermal leptogenesis for the both case which satisfies and breaks the SSA in the basis where $M_R$ is diagonal.

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arXiv2006

On the Fukugita-Tanimoto-Yanagida Ansatz with Partially Non-degenerate Right-handed Majorana Neutrinos

Taking three right-handed Majorana neutrino masses $M^{}_i$ to be partially non-degenerate, we make a new analysis of the Fukugita-Tanimoto-Yanagida ansatz and confront it with current neutrino oscillation data. We determine the parameter space for cases (A) $M^{}_3 = M^{}_2 \neq M^{}_1$, (B) $M^{}_2 = M^{}_1 \neq M^{}_3$ and (C) $M^{}_1 = M^{}_3 \neq M^{}_2$, and examine their respective deviations from the original $M^{}_1 = M^{}_2 = M^{}_3$ case. The numerical constraints on three light neutrino masses, three neutrino mixing angles and three CP-violating phases are also obtained, together with the predictions for the Jarlskog invariant of CP violation and the effective masses of the tritium beta decay and the neutrinoless double-beta decay.

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arXiv2006

Can symmetric texture reproduce unitarity triangle and $m_b/m_τ$ ?

We study the SUSY SO(10) GUT with the symmetric two-zero textures of quark/lepton mass matrix which realize the Georgei-Jarlskog relations in down-type quark and charged lepton masses. We show that the important constraints to such framework come from the bottom-tau unification and the observed value of $\sin2β$, one of the angles in the CKM unitarity triangle. We investigate the symmetric two-zero textures assumed at the GUT scale by solving the MSSM renormalization group equations with right-handed neutrino threshold effects. As a result, the value of $\tan\tildeβ$ is constrained to be large enough and the representation of Higgs field which couples to the third generation of left- and right-handed neutrinos is restricted by the bottom-tau unification condition. The textures with vanishing 1-2 (and 2-1) element for up-type quarks and vanishing 1-3 (and 3-1) element for down-type quarks are favored by the observation of $\sin2β$.

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arXiv2006

Generation Mixing of Sneutrinos in Heavier Chargino Decay

The heavier chargino decay could yield two charged leptons of different generations, owing to generation mixing of sneutrinos. We discuss the possibility of producing $e$ and $μ$ through this process in near future collider experiments. The analyses are made systematically in the supersymmetric extension of the standard model without assuming a specific scenario for the mixing. Production of the heavier chargino is evaluated in $e^+e^-$ collisions. In the parameter region consistent with nonobservation of the radiative $μ$ decay, sizable parts lead to a detectable branching ratio for the generation-changing decay of the heavier chargino.

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arXiv2004

Symmetric Mass Matrix with Two Zeros in SUSY SO(10) GUT, Lepton Flavor Violations and Leptogenesis

We study the symmetric 2-zero texture of the neutrino mass matrix, which is obtained from the symmetric Dirac neutrino mass matrix with 2-zeros and right-handed Majorana neutrino mass matrix with the general form via the see-saw mechanism, for the SUSY SO(10) GUT model including the Pati-Salam symmetry. We show that the only one texture in our model, having degenerate mass spectrum for the 1st and 2nd generation of right-handed Majorana neutrino, can simultaneously explain the current neutrino experimental data, lepton flavor violating processes and baryon asymmetry of the Universe. Within such a framework, the predicted values of the light and heavy Majorana neutrino masses, together with $|U_{e3}|$, $J_{\rm CP}$ and $|< M_{ee} >|$, are almost uniquely determined.

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arXiv2003

Can Symmetric Texture reproduce Neutrino Bi-large Mixing?

We study the symmetric texture of geometric form with 2-zeros to see if it is consistent with the presently-known neutrino masses and mixings. In the neutrino mass matrix elements we obtain numerically the allowed region of the parameters including CP violating phases, which can reproduce the present neutrino experiment data. The result of this analysis dictates the narrow region for the GUT model including Pati-Salam symmetry with texture zeros to be consistent with the experimental data. The $|U_{e3}|$ and $J_{CP}$ are also predicted in such models.

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arXiv2003

Neutrino Bi-Large Mixings and Family

After a brief review of quark-lepton relations in grand unified theories (GUT), we show that the Pati-Salam relation with only one type of Higgs field configuration with "four zero symmetric texture" can reproduce two large neutrino mixings as well as observed mass differences. This is quite in contrast to the case of SU(5) where bi-large mixings essentially come from the charged lepton sector with non-symmetric charged lepton mass matrix.

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arXiv2003

Neutrino Mass Matrix in Terms of Up-Quark Masses

We demonstrate that under the "symmetric zero texture" with minimal Majorana mass matrix, neutrino masses and mixing angles are expressed in terms of up-quark masses, $m_t,m_c,m_u$. This provides interesting relations among neutrino mixing angles and up-type quark masses. Especially we predict $|U_{e3}| \leq 0.11$ even if we include the small mixing effects coming from charged lepton side. Also absolute masses of three neutrinos are predicted almost uniquely. This is quite in contrast to the case where bi-large mixings come from the charged lepton sector with non-symmetric charged lepton mass matrix.

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arXiv2003

Neutrino Mass Matrix Predicted From Symmetric Texture

Within the framework of grand unified theories, we make full analysis of symmetric texture to see if such texture can reproduce large neutrino mixings, which have recently been confirmed by the observed solar and atmospheric neutrino oscillation experiments. It is found that so-called symmetric texture with anomalous U(1) family symmetry with Froggatt-Nielsen mechanism does not provide a natural explanation of two large mixing angles. On the contrary we should adopt "zero texture" which have been extensively studied by many authors and only this scenario can reproduce two large mixing angles naturally. Under such "zero texture" with minimal symmetric Majorana matrix, all the neutrino masses and mixing angles, 6 quantities, are expressed in terms of up-quark masses, $m_t,m_c,m_u$ with two adjustable parameters. This provides interesting relations among neutrio mixing angles, $\tan^2 2θ_{12} \simeq \frac{144m_c}{m_t} \tan^2 2θ_{23} \cos^2 θ_{23}, \quad \sin^2 θ_{13} \simeq \frac{4m_c}{m_t}\sin^2 θ_{23} \cos^2 θ_{12}$. Thus $|U_{e3}|$ is predicted to lie within the range $0.01-0.06$. Also absolute masses of three neutrinos are predicted almost uniquely. This is quite in contrast to the case where bi-large mixings come from the charged lepton sector with non-symmetric mass matrix, which does not provide the information of neutrino masses.

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arXiv2002

Neutrino Mass Matrix Out of Up-Quark Masses Only

Under the assumption of symmetric four zero texture for fermion mass matrices in SO(10) model, the neutrino Dirc mass matrix is derived from the up-quark mass matrix. By adjusting two scales of the Majorana mass sector, we can derive observed neutrino two large mixing angles very naturally. It is indeed remarkable that all the masses and mixings of neutrinos are expressed in terms of only three known parameters, $m_t,m_c,m_u$. A typical example of our model is \tan^2 2θ_{τμ}=\frac{4}{(1-2\frac{m_c}{\sqrt{m_u m_t}})^2}.

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arXiv2001

Mixing Matrix of Quarks Having Natural Twisting in E_6 Grand Unified Model

Mass matrix of quarks is studied in the Supersymmetric $E_6$ Grand Unified Theory (GUT). The fundamental representation $\textbf{27}$ in $E_6$ which corresponds to one generation contains two sets of $\textbf{5}^{*}$'s in SU(5), so that there are six flavors of lepton doublets and right-handed down-quark triplets. It is known that the twisting (interchange) among the $\textbf{5}^{*}$ representations may reproduce the observed quark and lepton mixing matrices. If $E_6$ is directly broken down to $SU(3)_C \times SU(2)_L \times U(1)_Y$ and two extra U(1)'s, the extra 3 sets of down-type quarks do not necessarily decouple from the quark mass matrix, under the appropriate choice of the U(1) flavor charges on the quark and Higgs fields. Then, by diagonalizing the 6 $\times$ 6 down-quark mass matrix, we find a certain set of parameters, in which the twisting between a pair of the right-handed down-quarks occurs naturally, reproducing the reasonable values for the $d$-, $s$- and $b$-quark masses, and for $V_{ud}$, $V_{us}$, $V_{cd}$, $V_{cs}$ and $V_{tb}$ of the CKM matrix elements. As a by-product, one vector-like down-quark appears at the experimentally accessible TeV scale.

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