Large Lepton Mixing in a Coset-space Family Unification on E7/SU(5)×U(1)3E_7/SU(5) \times U(1)^3

We study a coset-space unification model for families based on $E_7/SU(5) \times U(1)^3$. We find that qualitative structure of quark and lepton mass matrices in this model describes very well the observation. We stress, in particular, that the large mixing angle, $\sin^22\theta_{\nu_\mu\nu_\tau} \simeq 1$, required for the atmospheric neutrino oscillation reported by the SuperKamiokande collaboration, is naturally obtained, which is a consequence of unparallel family structure in the present coset-space unification.Comment: 8 pages, Latex2

A Solution to the Polonyi Problem in the Minimum SUSY-GUT

We show that the Polonyi problem is solved in the minimum SUSY-GUT model in which a self-coupling strength for a heavy Higgs $\Sigma$, $\lambda\Sigma^{3}$, is very small $\lambda\sim 10^{-6}$. It is stressed that with this small $\lambda$ the mass of the physical $\Sigma$ becomes m_{\Sigma} \sim 10^{12}\GEV and the unification scale is raised up to the gravitational one, M\simeq 2\times 10^{18}\GEV. A potential problem, however, is also pointed out in this GUT model

Supersymmetric Grand Unification Model with the Orbifold Symmetry Breaking in the Six Dimensional Supergravity

We construct supersymmetric (SUSY) grand unification (GUT) models in the six dimensional space-time where the GUT symmetry is broken down to the standard-model gauge group by a simple orbifolding T^2/Z_4 or T^2/Z_6 and a pair of massless Higgs doublets in the SUSY standard model are naturally obtained. Since the background geometry here is simple compared with models using the Scherk-Schwarz mechanism, one might hope for an approximate gauge coupling unification in the present models. Here, the presence of the massless Higgs multiplets in the bulk is quite natural, since the anomaly cancellation in the six dimensional space-time requires N=2 hyper multiplets in the bulk, some of which are origins of the Higgs doublets. However, the origin of the quarks and leptons is still not clear at all.Comment: 9 pages, including 2 figures, UT-96

A Solution to the Doublet-Triplet Splitting Problem in the Type IIB Supergravity

The doublet--triplet mass splitting problem is one of the most serious problems in supersymmetric grand unified theories (GUTs). A class of models based on a product gauge group, such as the SU(5)_{GUT} times U(3)_H or the SU(5)_{GUT} times U(2)_H, realize naturally the desired mass splitting that is protected by an unbroken R symmetry. It has been pointed out that various features in the models suggest that these product-group unification models are embedded in a supersymmetric brane world. We show an explicit construction of those models in the supersymmetric brane world based on the Type IIB supergravity in ten dimensions. We consider T^6/(Z_{12} times Z_2) orientifold for the compactified six extra dimensions. We find that all of the particles needed for the GUT-symmetry-breaking sector are obtained from the D-brane fluctuations. The three families of quarks and leptons are introduced at an orbifold singularity, although their origin remains unexplained. This paper includes extensive discussion on anomaly cancellation in a given orbifold geometry. Relation to the Type IIB string theory, realization of R symmetry as a rotation of extra-dimensional space, and effective superpotential at low energies are also discussed.Comment: 61 page

Higher Dimensional Supersymmetry as an Origin of the Three Families for Quarks and Leptons

In a (0,1) supersymmetric (SUSY) six-dimensional gauge theory, a gauge fermion gives rise to box anomalies. These anomalies are completely canceled by assuming a vector multiplet of (1,1) SUSY. With a T^2/Z_3 orbifold compactification of the extra two dimensional space, the theory provides three chiral multiplets and three equivalent fixed points. We regard them as the origin of the three families of quarks and leptons. Quasi anarchy structure in the SU(5)-5^* representation sector and hence the bimaximal mixing in the neutrino oscillation are explained quite naturally in this framework. We also discuss a family symmetry as a remnant of the higher-dimensional R symmetry.Comment: 12 pages, Contribution to "Peccei Fest" January 11-13,2002, a number of comments and an appendix are adde

Geometric Origin of Large Lepton Mixing in a Higher Dimensional Spacetime

The large mixing in the lepton sector observed in the recent neutrino-oscillation experiments strongly suggest that nature of left-handed lepton doublets is very different from that of left-handed quark doublets. This means that there is a big disparity between the matter multiplets 5^*'s and 10's in the SU(5) unified theory. We show that this big difference can be explained in a six-dimensional spacetime compactified on the T^2/Z_3 orbifold. That is, we propose to put three families of 5^*'s on three equivalent fixed points of the orbifold and the three 10's in the two-dimensional bulk. We construct an explicit model realizing this situation and show that the democratic mass structure in the lepton sector is naturally obtained and hence the model explains the observed bi-large lepton mixing and simultaneously the required small mixing U_e3. The mass matrices and mixing in the quark sector are also briefly discussed.Comment: 12 pages, including 1 figure and 1 table, one paragraph and two references adde

Quark Lepton Mass Hierarchies and the Baryon Asymmetry

The mass hierarchies of quarks and charged leptons as well as a large \n_\m-\n_\t mixing angle are naturally explained by the Frogatt-Nielsen mechanism with a nonparallel family structure of chiral charges. We extend this mechanism to right-handed neutrinos. Their out-of-equilibrium decay generates a cosmological baryon asymmetry whose size is quantized in powers of the hierarchy parameter \e^2. For the simplest hierarchy pattern the neutrino mass \bar{m}_\n= (m_{\n_\m}m_{\n_\t})^{1/2} \sim 10^{-2} eV, which is inferred from present indications for neutrino oscillations, implies a baryon asymmetry $n_B/s \sim 10^{-10}$. The corresponding baryogenesis temperature is $T_B \sim 10^{10}$ GeV.Comment: 7 page

511 keV Gamma Ray from Moduli Decay in the Galactic Bulge

We show that the $e^++e^-$ decay of a light scalar boson of mass 1-10 MeV may account for the fluxes of 511 keV gamma ray observed by SPI/INTEGRAL. We argue that candidates of such a light scalar boson is one of the string moduli or a scalar partner of the axion in a supersymmetric theory.Comment: 6 pages, no figure, we have added an appendix to describe the dilution of the moduli by the thermal inflation and some minor correction