Possible Flavor Mixing Structures of Lepton Mass Matrices

To search for possible textures of lepton mass matrices, we systematically examine flavor mixing structures which can lead to large lepton mixing angles. We find out 37 mixing patterns are consistent with experimental data, taking into account phase factors in the mixing matrices. Only six of the patterns can explain the observed data without any tuning of parameters, while the others need particular choices for the phase values. It is found that these six mixing patterns are those predicted by the models which have been proposed to account for fermion mass hierarchies. On the other hand, the others may give new flavor mixing structures of lepton mass matrices and therefore new possibilities of model construction.Comment: 21 page

Neutrino mass matrix with U(2) flavor symmetry and neutrino oscillations

The three neutrino mass matrices in the $SU(5)\times U(2)$ model are studied focusing on the neutrino oscillation experiments. The atmospheric neutrino anomaly could be explained by the large $\nu_{\mu} - \nu_{\tau}$ oscillation. The long baseline experiments are expected to detect signatures of the neutrino oscillation even if the atmospheric neutrino anomaly is not due to the neutrino oscillation. However, the model cannot solve the solar neutrino deficit while it could be reconciled with the LSND data.Comment: 12 pages, LaTex file, to be published in PR

Is CP Violation Observable in Long Baseline Neutrino Oscillation Experiments ?

We have studied CP violation originated by the phase of the neutrino mixing matrix in the long baseline neutrino oscillation experiments. The direct measurements of CP violation is the difference of the transition probabilities between CP-conjugate channels. In those experiments, the CP violating effect is not suppressed if the highest neutrino mass scale is taken to be 1\sim 5 \eV, which is appropriate for the cosmological hot dark matter. Assuming the hierarchy for the neutrino masses, the upper bounds of CP violation have been caluculated for three cases, in which mixings are constrained by the recent short baseline ones. The calculated upper bounds are larger than $10^{-2}$, which will be observable in the long baseline accelerator experiments. The matter effect, which is not CP invariant, has been also estimated in those experiments.Comment: 28 pages, LaTex file, 6 figures included using epsfig Matter effect is estimated(Figs.3(a) (b)). Physical parameters are change

Deviation of Neutrino Mixing from Bi-maximal

We have studied how observables of the neutrino mixing matrix can link up with the ones in the quark sector. The deviation from the bi-maximal flavor mixing is parameterized using a 3 x 3 unitary matrix. The neutrino mixings are investigated supposing this unitary matrix to be hierarchical like the quark mixing matrix. We obtain the remarkable prediction |U_{e3}| >= 0.03 from the experimentally allowed range tan^2 theta_{sol} = 0.24 ~ 0.89. The CP violation in neutrino oscillations is expected to be very small.Comment: Some references are adde

Natural Neutrino Mass Matrix

Naturalness of the neutrino mass hierarchy and mixing is studied. First we select among 12 neutrino mixing patterns a few patterns, which could form the natural neutrino mass matrix. Further we show that if the Dirac neutrino mass matrix is taken as the natural one in the quark sector, then only two mixing patterns without the large mixing lead to the natural right-handed Majorana mass matrix. The rest of the chosen patterns with three degenerate mass solution lead to the unnatural right-handed Majorana mass matrix in the see-saw mechanism. Notice however, that for the chosen two natural patterns there could be a huge mass hierarchy such as ${\cal O}(10^{4\sim 6})$ in order to reproduce the inverse mass hierarchy of the light neutrinos.Comment: 31 pages, LaTex file, no figures, arguments made more clear, main conclusions unchanged, version accepted for publication in PRD Reort-no: Lund-Mph-97/14 Revise

CP Violation in Bilarge Lepton Mixing

We propose a scheme of lepton mixing in which the unitary matrix that diagonalizes the neutrino mass matrix is bimaximal and the deviation from bimaximal of the lepton mixing matrix is due to the unitary matrix that diagonalizes the charged-lepton mass matrix. This matrix is assumed to be hierarchical, like the quark mixing matrix. It is shown that in general it is possible to have a sizable value for |U_{e3}| together with an effective two-neutrino maximal mixing in solar neutrino experiments. If the effective mixing in solar neutrino experiments is less than maximal, as indicated by current data, |U_{e3}| is bounded from below. Furthermore, in general the violation of CP could be relatively large.Comment: 11 pages. Final version to be published in PR

Lepton Flavor Violating Process in Bi-maximal texture of Neutrino Mixings

We investigate the lepton flavor violation in the framework of the MSSM with right-handed neutrinos taking the large mixing angle MSW solution in the quasi-degenerate and the inverse-hierarchical neutrino masses. We predict the branching ratio of $\mu \to e+\gamma$ and $\tau \to \mu+\gamma$ processes assuming the degenerate right-handed Majorana neutrino masses. We find that the branching ratio in the quasi-degenerate neutrino mass spectrum is 100 times smaller than the ones in the inverse-hierarchical and the hierarchical neutrino spectra. We emphasize that the magnitude of $U_{e3}$ is one of important ingredients to predict BR($\mu \to e +\gamma$). The effect of the deviation from the complete-degenerate right-handed Majorana neutrino masses are also estimated. Furtheremore, we examine the S_{3\sL}\times S_{3\sR} model, which gives the quasi-degenerate neutrino masses, and the Shafi-Tavartkiladze model, which gives the inverse-hierarchical neutrino masses. Both predicted branching ratios of $\mu\to e+\gamma$ are smaller than the experimantal bound.Comment: Latex file, 38 pages, 10 figures, revised versio

Lepton Flavor Model from Delta(54) Symmetry

We present the lepton flavor model with $\Delta (54)$, 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 $\sin^2\theta_{13}$ is 0.01. There is the strong correlation between $\theta_{23}$ and $\theta_{13}$. Unless $\theta_{23}$ is deviated from the maximal mixing considerably, $\theta_{13}$ remains to be tiny.Comment: 19 pages, 7 figures; some discussions are modifie

Leptogenesis in Neutrino Textures with Two Zeros

The leptogenesis is studied in the neutrino textures with two zeros, which reduce the number of independent phases of the CP violation. The phenomenological favored neutrino textures with two zeros are decomposed into the Dirac neutrino mass matrix and the right-handed Majorana one in the see-saw mechanism. Putting the condition to suppress the $\mu \to e\gamma$ decay enough, the texture zeros of the Dirac neutrino mass matrix are fixed in the framework of the MSSM with right-handed neutrinos. These textures have only one CP violatig phase. The magnitude of each entry of the Dirac mass matrix is determined in order to explain the baryon asymmetry of the universe by solving the Boltzman equations. The relation between the leptogenesis and the low energy CP violation is presented in these textures.Comment: Latex file with 20 pages, 6 eps figure

Explicit CPCP Violation of the Higgs Sector in the Next-to-Minimal Supersymmetric Standard Model

We study the explicit $CP$ violation of the Higgs sector in the minimal supersymmetric standard model with a gauge singlet Higgs field. The magnitude of $CP$ violation is discussed in the limiting cases of $x\gg v_1,\ v_2$ and $x\ll v_1,\ v_2$, where $x$ and $v_{1,2}$ denote VEV of singlet and doublet Higgs scalars, respectively. Our numerical predictions of the electric dipole moments of electron and neutron lie around the present experimental upper limits. It is found that the large $CP$ violation effect reduces the magnitude of the lightest Higgs boson mass in the order of a few ten GeV.Comment: 20 pages, LATEX, 5 figures(not included, available on request