[en] We study physics potential of Very Long Base-Line (VLBL) Neutrino-Oscillation Experiments with the High Intensity Proton Accelerator, which will be completed by the year 2007 in Tokai-village, Japan. As a target, a 100 kton-level water-Cerenkov detector is considered at 2,100 km away. Assuming the pulsed narrow-band ν_μ beams, we study sensitivity of such experiments to the neutrino mass hierarchy, the mass-squared differences, one CP phase and three angles of the lepton-flavor-mixing matrix. We find that experiments at a distance 2,100 km can determine the neutrino mass hierarchy if the mixing matrix element vertical bar U_e3 vertical bar is not too small. The CP phase and vertical bar U_e3 vertical bar can be constrained if the large-mixing-angle solution of the solar-neutrino deficit is realized

[en] We study the sensitivity of a long-base-line (LBL) experiment with neutrino beams from the High Intensity Proton Accelerator (HIPA), that delivers 10²¹ POT per year, and a proposed 1 Mt water Cerenkov detector, Hyper-Kamiokande (HK) 295 km away from the HIPA, to the CP phase (δ_MNS) of the three-flavor lepton mixing matrix. We examine a combination of the ν_μ narrow-band beam (NBB) at two different energies, < p_π>=2, 3 GeV, and the ν-bar_μ NBB at < p_π>=2 GeV. By allocating one year each for the two ν_μ beams and four years for the ν-bar_μ beam, we can efficiently measure the ν_μ→ν_e and ν-bar_μ→ν-bar_e transition probabilities, as well as the ν_μ and ν-bar_μ survival probabilities. CP violation in the lepton sector can be established at 4σ (3σ) level if the MSW large-mixing-angle scenario of the solar-neutrino deficit is realized, vertical bar δ_MNS vertical bar or vertical bar δ_MNS-180 deg. vertical bar >30 deg., and if 4 vertical bar U_e3 vertical bar²(1-vertical bar U_e3 vertical bar²)≡sin²2θ_RCT>0.03 (0.01). The phase δ_MNS is more difficult to constrain by this experiment if there is little CP violation, δ_MNS∼0 deg. or 180 deg., which can be distinguished at 1σ level if sin²2θ_RCT∼>0.01

[en] We study a rate asymmetry in the inclusive decay B → X_sγ, assuming the supersymmetric standard model based on N = 1 supergravity. A complex coefficient A for scalar-trilinear couplings is the source of CP violation, which is contained in the mass-squared matrices for squarks. The model parameters are non-trivially constrained by experimental results for the branching ratio of the radiative decay and for the electric dipole moments of the neutron and the electron. The decay rate asymmetry is predicted to be much larger than that by the standard model in a wide region of the parameter space compatible with the experiments. Its magnitude can be maximally around 0.1, which will be well accessible at B factories

[en] We discuss a rate asymmetry for the radiative B-meson decay B→X_sγ within the framework of the supersymmetric standard model based on N=1 supergravity. This model contains new sources of CP violation. In spite of severe experimental constraints on the electric dipole moment of the neutron, a new CP-violating phase may not be suppressed, leading to a sizeable enhancement of the decay rate asymmetry. The magnitude of the asymmetry is predicted to be larger than that from the standard model in wide parameter regions where the branching ratio is consistent within experimental bounds. A possible maximal magnitude is about 0.1, which will be well accessible at B factories. (author)

[en] Current searches for doubly charged Higgs bosons (H^±±) at the Fermilab Tevatron are sensitive to single production of H^±±, although the pair production mechanism qq→H⁺⁺H^-- is assumed to be dominant. In the context of a Higgs Triplet Model we study the mechanism q^'q→H^±±H^± at the Tevatron and CERN Large Hadron Collider, and show that its inclusion can significantly improve the search potential for H^±±. Moreover, assuming that the neutrino mass is generated solely by the triplet field Yukawa coupling to leptons, we compare the branching ratios of H^±±→l^±l^± and H^±±→H^±W* for the cases of a normal hierarchical, inverted hierarchical, and degenerate neutrino mass spectrum

No abstract available

[en] We discuss a supersymmetric extension of the standard model with an extra U(1) gauge symmetry. In this model, the proton stability is guaranteed by the gauge symmetry without invoking R parity. The gauge symmetry breakdown automatically generates an effective μ term and large Majorana masses for right-handed neutrinos. The supersymmetry-soft-breaking terms for scalar fields could be universal at a very high energy scale, and the electroweak symmetry is broken through radiative corrections. (author)

[en] We propose a hybrid seesaw model based on $A_4$ flavor symmetry, which generates a large hierarchical flavor structure. In our model, tree-level and one-loop seesaw mechanisms predict different flavor structures in the neutrino mass matrix and generate a notable hierarchy among them. We find that such a hierarchical structure gives a large effective neutrino mass that can be accessible by next-generation neutrinoless double beta decay experiments. Majorana phases can also be predictable. The $A_4$ flavor symmetry in the model is spontaneously broken to the $Z_2$ symmetry, leading to a dark matter candidate that is assumed to be a neutral scalar field. The favored mass region of the dark matter is obtained by numerical computations of the relic abundance and the cross section of the nucleon. We also investigate the predictions of several hierarchical flavor structures based on $A_4$ symmetry for the effective neutrino mass and the Majorana phases, and find characteristic features depending on the hierarchical structures.

[en] We study bounds on Higgs-boson masses from perturbative unitarity in the Georgi-Machacek model, whose Higgs sector is composed of a scalar isospin doublet and a real and a complex isospin triplet field. This model can be compatible with the electroweak precision data without fine-tuning because of the imposed global SU(2)_R symmetry in the Higgs potential, by which the electroweak rho parameter is unity at the tree level. All possible two-body elastic-scattering channels are taken into account to evaluate the S-wave amplitude matrix, and then the condition of perturbative unitarity is imposed on the eigenvalues to obtain constraint on the Higgs parameters. Masses of all scalar bosons turn out to be bounded from above, some of which receive more strict upper bounds as compared to that in the standard model (712 GeV). In particular, the upper bound of the lightest scalar boson, whatever it would be, is about 270 GeV.

[en] We investigate a ℤ_3 symmetric model with two-loop radiative neutrino masses. Dark matter in the model is either a Dirac fermion or a complex scalar as a result of an unbroken ℤ_3 symmetry. In addition to standard annihilation processes, semi-annihilation of the dark matter contributes to the relic density. We study the effect of the semi-annihilation in the model and find that those contributions are important to obtain the observed relic density. The experimental signatures in dark matter searches are also discussed, where some of them are expected to be different from the signatures of dark matter in ℤ_2 symmetric models