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Teruyuki Kitabayashi

Publications and source records attributed to Teruyuki Kitabayashi.

At least 55 records · Page 3Linked to original sources
arXiv2016

Maximal CP Violation in Flavor Neutrino Masses

Since flavor neutrino masses $M_{μμ,ττ,μτ}$ can be expressed in terms of $M_{ee,eμ,eτ}$, mutual dependence among $M_{μμ,ττ,μτ}$ is derived by imposing some constraints on $M_{ee,eμ,eτ}$. For appropriately imposed constraints on $M_{ee,eμ,eτ}$, we show a texture of neutrino mass matrix giving rise to maximal CP violation. When the atmospheric neutrino mixing is also maximal, we discuss various specific textures of neutrino mass matrices including the texture with $M_{ττ}=M^\ast_{μμ}$ derived as the simplest solution to the constraint of $M_{ττ}-M_{μμ}$=imaginary, which is required by the constraint of $M_{eμ}\cosθ_{23}-M_{eτ}\sinθ_{23}$=real for $\cos 2θ_{23}=0$. It is found that Majorana CP violation depends on the phase of $M_{ee}$.

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arXiv2016

Asymmetric dark matter and effective number of neutrinos

We study the effect of the MeV-scale asymmetric dark matter annihilation on the effective number of neutrinos $N_{\rm eff}$ at the epoch of the big bang nucleosynthesis. If the asymmetric dark matter $χ$ couples more strongly to the neutrinos $ν$ than to the photons $γ$ and electrons $e^-$, $Γ_{χγ, χe} \ll Γ_{χν}$, or $Γ_{χγ, χe} \gg Γ_{χν}$, the lower mass limit on the asymmetric dark matter is about $18$ MeV for $N_{\rm eff}\simeq 3.0$.

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arXiv2014

Parameterization of Pontecorvo-Maki-Nakagawa-Sakata mixing matrix based on CP-violating bipair neutrino mixing

CP violation in neutrino interactions is described by three phases contained in Pontecorvo-Maki-Nakagawa-Sakata mixing matrix ($U_{PMNS}$). We argue that the phenomenologocally consistent result of the Dirac CP violation can be obtained if $U_{PMNS}$ is constructed along bipair neutrino mixing scheme, namely, requiring that $ | U_{12} | = | U_{32} | {\rm and} | U_{22} | = | U_{23} | (\rm case 1)$ and $ | U_{12} | = | U_{22} | {\rm and} | U_{32} | = | U_{33}| (\rm case 2)$, where $U_{ij}$ stands for the $i$-$j$ matrix element of $U_{PMNS}$. As a results, the solar, atmospheric and reactor neutrino mixing angles $θ_{12}$, $θ_{23}$ and $θ_{13}$, respectively, are correlated to satisfy $\cos 2{θ_{12}} = \sin^2θ_{23} - \tan^2θ_{13}$ (case 1) or $\cos 2{θ_{12}} = \cos^2θ_{23} - \tan^2θ_{13}$ (case 2). Furthermore, if Dirac CP violation is observed to be maximal, $θ_{23}$ is determined by $θ_{13}$ to be: $\sin^2θ_{23} \approx ({\sqrt 2 - 1})({\cos^2θ_{13} + \sqrt 2 \sin^2θ_{13}})$ (case 1) or $\cos^2θ_{23} \approx ({\sqrt 2 - 1})({\cos^2θ_{13} + \sqrt 2 \sin^2θ_{13}})$ (case 2). For the case of non-maximal Dirac CP violation, we perform numerical computation to show relations between the CP-violating Dirac phase and the mixing angles.

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arXiv2014

Fermi-Boltzmann statistics of neutrinos and relativistic effective degrees of freedom

We investigate the effect of the presence of non-pure fermionic neutrinos on the relativistic effective degrees of freedom in the early universe. The statistics of neutrinos is transformed continuously from Fermi-Dirac to Maxwell-Boltzmann statistics. We find that the relativistic degrees of freedom decreases with the deviation from pure Fermi-Dirac statistics of neutrinos if there are constant and large lepton asymmetries. Additionally, we confirm that the change of the statistics of neutrinos from Fermi-Dirac to Maxwell-Boltzmann is not sufficient to cover the excess of the effective number of neutrinos.

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arXiv2014

CP violation in modified bipair neutrino mixing and leptogenesis

We study effects of CP violation in a modified bipair neutrino mixing scheme predicting $\sin^2θ_{23}$ near both 0.4 and 0.6 currently consistent with experimentally allowed values. The source of CP violation is supplied by charged lepton mixing accompanied by a single phase, whose mixing size is assumed to be less than that of the Wolfenstein parameter for the quark mixing. Including results of leptogenesis, which is based on the minimal seesaw model, we obtain the allowed region of CP-violating Dirac and Majorana phases, which provides the observed baryon asymmetry of the universe in the case of the Dirac neutrino mass matrix subject to one zero texture.

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arXiv2013

CP violation in bipair neutrino mixing

There are experimentally determined two best-fit points for the atmospheric neutrino mixing angle $θ_{23}$: $\sin^2 θ_{23} = 0.413$ (case A) and $\sin^2 θ_{23} = 0.594$ (case B). In the bipair neutrino mixing scheme, we predict $\sin^2θ_{23} = \sqrt{2}-1$ (case 1) to be consistent with the case A and $\sin^2θ_{23} = 2-\sqrt{2}$ (case 2) to be consistent with the case B. If the case B is realized in nature, the bipair neutrino mixing provides a unique neutrino model consistent with the observation $\sin^2 θ_{23} = 0.594$. However, the reactor neutrino mixing angle $θ_{13}$ is predicted to be $\sin^2θ_{13} = 0$, which is inconsistent with the observation. We propose a new modification scheme to yield $\sin^2θ_{13} \neq 0$ utilizing the charged lepton contribution and study its effect on both of CP-violating Dirac and Majorana phases, which is numerically estimated. It is found that there appear striking differences between the case 1 and the case 2 in their phase structure.

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arXiv2012

Charged lepton contributions to bipair neutrino mixing

The bipair neutrino mixing describes the observed solar and atmospheric mixings; however, it predicts vanishing reactor mixing angle, which is inconsistent with the observed data. We explore the ways of minimally modifying the bipair neutrino mixing by including charged lepton contributions. There are two categories of the bipair neutrino mixing which are referred to as case 1 and case 2. It turns out that, without arbitrary phases, a minimal modification is realized by just considering one $e$ - $τ$ contribution from the charged lepton sector in the case 1. On the other hand, not only $e$ - $τ$ contribution but also $μ$ - $τ$ contribution is required to realize a minimal modification in the case 2.

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arXiv2011

Majorana CP phases in bi-pair neutrino mixing and leptogenesis

We estimate Majorana CP phases for a given flavor neutrino mass matrix (M_ν) consistent with the bi-pair neutrino mixing, which is recently proposed to describe neutrino mixings given by \sinθ_{13}=0 for the reactor neutrino mixing, \sin^2θ_{12} = 1-1/\sqrt{2} for the solar neutrino mixing and either \sin^2θ_{23} = \tan^2θ_{12} or \sin^2θ_{23}=1-\tan^2θ_{12} for the atmospheric neutrino mixing. Sizes of Majorana CP phases are evaluated so as to generate the observed baryon asymmetry in the universe via a leptogenesis scenario within the framework of the minimal seesaw model, where M_νsatisfies det(M_ν)=0 and one active Majorana CP phase (ϕ) is present. Assuming the normal mass hierarchy for light neutrinos and one zero texture for a 3X2 Dirac neutrino mass matrix, we find that ϕlies in the region of 0.69<|ϕ|<0.92 [rad], which is converted into allowed regions of α=arg(M_{eμ}) and β=arg(M_{eτ}), where M_{ij} (i,j=e,μ,τ) denote the i-j matrix element of M_ν. The phases αand βturn out to satisfy 0.31<|α|<0.40 [rad] and -1.25<β<-0.32 [rad]. The approximate numerical equality of |ϕ|\approx 2|α| is consistent with our theoretical estimation of ϕ=ϕ_2-ϕ_3 for ϕ_2=-(α+β) and ϕ_3\approx α-βvalid for the normal mass hierarchy. We also find the following scaling property: (M'^{μμ}-M'^{ee}/t^2_{12})/M'^{μτ}=M'^{μτ}/(M'^{ττ}-M'^{ee}/t^2_{12})=-M'^{eτ}/M'^{eμ} (t^2_{12}=\tan^2θ_{12}=\sqrt{2}-1), where M'^{ij} stands for M_{ij} evaluated on the basis of the Particle Data Group's phase convention.

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arXiv2011

Bi-pair Neutrino Mixing

A new type of neutrino mixing named bi-pair neutrino mixing is proposed to describe the current neutrino mixing pattern with a vanishing reactor mixing angle and is determined by a mixing matrix with two pairs of identical magnitudes of matrix elements. As a result, we predict \sin^2θ_{12}=1-1/\sqrt{2}(\approx 0.293) for the solar neutrino mixing and either \sin^2θ_{23}=\tan^2θ_{12} or \cos^2θ_{23}=\tan^2θ_{12} for the atmospheric neutrino mixing. We determine flavor structure of a mass matrix M, leading to diagonal masses of m_{1,2,3}, and find that |M_{μμ}-M_{ee}/t^2_{12}|:|M_{μτ}|:|M_{ττ}-M_{ee}/t^2_{12}|=t^2_{23}:|t_{23}|:1 for the normal mass hierarchy if m_1=0, where t_{ij}=\tanθ_{ij} (i,j=1,2,3) and M_{ij} (i,j=e,μ,τ) stand for flavor neutrino masses. For the inverted mass hierarchy, the bi-pair mixing scheme turns out to satisfy the strong scaling ansatz requiring that |M_{μμ}|:|M_{μτ}|:|M_{ττ}|=1:|t_{23}|:t^2_{23} if m_3=0.

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arXiv2007

Remark on the minimal seesaw model and leptogenesis with tri/bi-maximal mixing

We have studied a leptogenesis scenario in the framework of the minimal seesaw model with tri/bi-maximal mixing. Usually, at least one of the elements in the Dirac mass matrix is fixed to be zero, for example, we denote it by $b_2=0$. We have pointed out that the absolute value of the CP asymmetry has several minimums and maximums with non-zero $b_2$. Thus one can expect that more rich phenomena, such as an enhanced leptogenesis, are hidden in the $b_2 \neq 0$ space.

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arXiv2005

Determination of Neutrino Mass Texture for Maximal CP Violation

We show a general form of neutrino mass matrix (M), whose matrix elements are denoted by M_{ij} (i.j=e, mu, tau) as flavor neutrino masses, that induces maximal CP violation as well as maximal atmospheric neutrino mixing. The masses of M_{mu mu}, M_{tau tau} and M_{mu tau}+sigma M_{ee} (sigma =\pm 1) turn out to be completely determined by M_{e mu} and M_{e tau} for given mixing angles. The appearance of CP violation is found to originate from the interference between the mu-tau symmetric part of M and its breaking part. If |M_{e mu}|=|M_{e tau}|, giving either M_{e mu}=-sigma e^{i theta} M_{e tau} or M_{e mu}=-sigma e^{i theta}M*_{e tau} with a phase parameter theta, is further imposed, we find that |M_{mu mu}|=|M_{tau tau}|is also satisfied. These two constraints can be regarded as an extended version of the constraints in the mu-tau symmetric texture given by M_{e mu}=-sigma M_{e tau} and M_{mu mu}=M_{tau tau}. Majorana CP violation becomes active if arg(M_{mu tau})\neq arg(M_{e mu})+theta/2 for M_{e mu}=-sigma e^{i theta} M_{e tau} and if arg(M_{mu tau})\neq theta/2 for M_{e mu}=-sigma e^{i theta} M*_{e tau}.

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arXiv2005

Neutrino Mass Textures with Maximal CP Violation

We have found three types of neutrino mass textures, which give maximal CP-violation as well as maximal atmospheric neutrino mixing. These textures are described by six real mass parameters: one specified by two complex flavor neutrino masses and two constrained ones and the others specified by three complex flavor neutrino masses. In each texture, we calculate mixing angles and masses as well as Majorana CP phases.

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arXiv2005

μ-τSymmetry and Maximal CP Violation

We argue the possibility that a real part of a flavor neutrino mass matrix only respects a mu-tau symmetry. This possibility is shown to be extended to more general case with a phase parameter θ, where the mu-tau symmetric part has a phase of θ/2. This texture shows maximal CP violation and no Majorana CP violation.

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arXiv2005

Constraints on Flavor Neutrino Masses and sin^2(2theta_{12})>>sin^2(theta_{13}) in Neutrino Oscillations

To realize the condition of sin^2(2theta_{12})>>sin^2(theta_{13}), we find constraints on flavor neutrino masses M_{ij} (ij=e,μ,τ): C1) c_{23}^2 M_{μμ} + s_{23}^2 M_{ττ} \approx 2 s_{23} c_{23}M_{μτ} + M_{ee} and/or C2) |c_{23}M_{eμ} -s_{23}M_{eτ}|>> |s_{23}M_{eμ} +c_{23}M_{eτ}|, where c_{23}=cos(theta_{23}) (s_{23}=sin(theta_{23})) and theta_{12}, theta_{13} and theta_{23} are the mixing angles for three flavor neutrinos. The applicability of C1) and C2) is examined in models with one massless neutrino and two massive neutrinos suggested by \det(M)=0, where M is a mass matrix constructed from M_{ij} (i,j=e,μ,τ). To make definite predictions on neutrino masses and mixings, especially on sin(theta_{13}), that enable us to trace C1) and C2), M is assumed to possess texture zeros or to be constrained by textures with M_{μμ}=M_{ττ} or M_{eτ}=\pm M_{eμ} which turn out to ensure the emergence of the maximal atmospheric neutrino mixing at sin(theta_{13})->0. It is found that C1) is used by textures such as M_{eμ}=0 or M_{eτ}=0 while C2) is used by textures such as M_{eτ}=\pm M_{eμ}.

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arXiv2004

Bilarge Neutrino Mixing and μ- τPermutation Symmetry for Two-loop Radiative Mechanism

The presence of approximate electron number conservation and μ-τpermutation symmetry of S_2 is shown to naturally provide bilarge neutrino mixing. First, the bimaximal neutrino mixing together with U_{e3}=0 is guaranteed to appear owing to S_2 and, then, the bilarge neutrino mixing together with |U_{e3}|<<1 arises as a result of tiny violation of S_2. The observed mass hierarchy of Δm^2_{\odot}<<Δm^2_{atm} is subject to another tiny violation of the electron number conservation. This scenario is realized in a specific model based on SU(3)_L x U(1)_N with two-loop radiative mechanism for neutrino masses. The radiative effects from heavy leptons contained in lepton triplets generate the bimaximal structure and those from charged leptons, which break S_2, generate the bilarge structure together with |U_{e3}|<<1. To suppress dangerous flavor-changing neutral current interactions due to Higgs exchanges especially for quarks, this S_2 symmetry is extended to a discrete Z_8 symmetry, which also ensures the absence of one-loop radiative mechanism.

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arXiv2002

$S_{2L}$ permutation symmetry for left-handed $μ$ and $τ$ families and neutrino oscillations in an $SU(3)_L \times U(1)_N$ gauge model

We construct an $SU(3)_L \times U(1)_N$ gauge model based on an $S_{2L}$ permutation symmetry for left-handed $μ$ and $τ$ families, which provides the almost maximal atmospheric neutrino mixing and the large solar neutrino mixing of the LMA type. Neutrinos acquire one-loop radiative masses induced by the radiative mechanism of the Zee type as well as tree level masses induced by the type II seesaw mechanism utilizing interactions of lepton triplets with an SU(3)-sextet scalar. The atmospheric neutrino mixing controlled by the tree-level and radiative masses turns out to be almost maximal owing to the presence of $S_{2L}$ supplemented by a $Z_4$ discrete symmetry. These symmetries ensure the almost equality between the $ν_e$-$ν_μ$ and $ν_e$-$ν_τ$ radiative masses dominated by contributions from heavy leptons contained in the third members of lepton triplets, whose Yukawa interactions conserve $S_{2L}$, even after the spontaneous breaking. The solar neutrino mixing controlled by radiative masses including a $ν_μ$-$ν_τ$ mass, which are taken to be the similar order, turns out to be described by large solar neutrino mixing angles.

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arXiv2001

Large Solar Neutrino Mixing and Radiative Neutrino Mechanism

We find that the presence of a global $L_e-L_μ-L_τ$ ($\equiv L^\prime$) symmetry and an $S_2$ permutation symmetry for the $μ$- and $τ$-families supplemented by a discrete $Z_4$ symmetry naturally leads to almost maximal atmospheric neutrino mixing and large solar neutrino mixing, which arise, respectively, from type II seesaw mechanism initiated by an $S_2$-symmetric triplet Higgs scalar $s$ with $L^\prime=2$ and from radiative mechanism of the Zee type initiated by two singly charged scalars, an $S_2$-symmetric $h^+$ with $L^\prime=0$ and an $S_2$-antisymmetric $h^{\prime +}$ with $L^\prime=2$. The almost maximal mixing for atmospheric neutrinos is explained by the appearance of the democratic coupling of $s$ to neutrinos ensured by $S_2$ and $Z_4$ while the large mixing for solar neutrinos is explained by the similarity of $h^+$- and $h^{\prime +}$-couplings described by $f^h_+\sim f^h_-$ and $μ_+\simμ_-$, where $f^h_+$ ($f^h_-$) and $μ_+$ ($μ_-$) stand for $h^+$ ($h^{\prime +}$)-couplings, respectively, to leptons and to Higgs scalars.

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