Proton Stability and Small Neutrino Mass in String Inspired E6E_6 Models

We propose a new possibility to realize simultaneously the sufficient proton stability and the interesting structure of neutrino mass matrix in superstring inspired $E_6$ models. In this model the leptons and Higgs fields are assigned to a fundamental representation {\bf 27} in the different way among generations. Two pairs of Higgs doublets naturally remain light from three generation ones by imposing certain discrete symmetries, although all extra color triplets become sufficiently heavy. Under these symmetries suitable $\mu$-terms to bring appropriate vacuum expectation values are prepared and the dangerous FCNC is avoidable. Some related phenomena to this model, especially, the structure of neutrino mass matrix are also discussed.Comment: 20 pages, latex, a version published in Prog. Theor. Phys., discussions related to neutrino masses are change

Strong CP-Problem in Superstring Theory

We apply the solution for the strong CP-problem in the 4-dimensional superstring theory recently proposed by Ib${\rm\acute{a}\tilde{n}}$ez and L${\rm\ddot{u}}$st to Calabi-Yau type models and study its phenomenological aspects. In Calabi-Yau type models there seem to be phenomenologically difficult problems in the axion decoupling from the neutral gauge currents and the compatibility between the proton stability and the cosmological bound on the axion. DFSZ type invisible axion mechanism which works without heavy extra colored fields may be more promising than KSVZ axion in the viewpoint of proton stability.Comment: 11 pages, DPKU920

Thermal leptogenesis in a TeV scale model for neutrino masses

It is known that the radiative neutrino mass model proposed by Ma could be a consistent framework for dark matter, leptogenesis and suppressed lepton flavor violation if a neutral component of the inert doublet is identified as dark matter and the right-handed neutrinos are of O(10^7) GeV or more. In the same model we explore another scenario such that right-handed neutrinos are in TeV regions and their lightest one is dark matter. It is shown that this scenario requires fine mass degeneracy to generate the appropriate baryon number asymmetry as in the case of resonant leptogenesis. As long as we impose the model to induce the baryon number asymmetry on the basis of thermal leptogenesis, we find that dark matter abundance can not be explained. If this scenario is adopted, the model has to be extended to include some new mechanism to explain it.Comment: 21 pages, 3 figures, v2:large improvement by the analysis of Boltzmann equations, title changed, v3: abstract rewritten, some references added, accepted version for publicatio

The origin of quark and lepton mixings

We propose a model for the mass matrices of quarks and leptons based on two Abelian flavor symmetries. One is assumed to be broken at a high energy region near the Planck scale. It is used for the Froggatt-Nielsen mechanism in both quark and charged lepton sectors. Another one remains unbroken to a multi-TeV region. The mixings among neutrinos and gauginos including the one of the new Abelian symmetry generate non-zero masses and mixings among neutrinos. A bi-maximal scheme for the neutrino oscillation can be realized together with suitable masses and CKM-mixings in the quark sector. FCNC constraints on this flavor dependent Abelian symmetry seems to be evaded.Comment: 18 pages, latex, one eps figur

Leptogenesis in a neutrino mass model coupled with inflaton

We propose a scenario for the generation of baryon number asymmetry based on the inflaton decay in a radiative neutrino mass model extended with singlet scalars. In this scenario, lepton number asymmetry is produced through the decay of non-thermal right-handed neutrinos caused from the inflaton decay. Since the amount of non-thermal right-handed neutrinos could be much larger than the thermal ones, the scenario could work without any resonance effect for rather low reheating temperature. Sufficient baryon number asymmetry can be generated for much lighter right-handed neutrinos compared with the Davidson-Ibarra bound.Comment: 14 pages, 2 figure

An extension of the SM based on effective Peccei-Quinn Symmetry

Peccei-Quinn (PQ) mechanism based on a chiral global U(1) symmetry is considered to be a simple and elegant solution for strong CP problem. Fact that the mechanism could be experimentally examined through the axion search makes it much more interesting and recently it causes a lot of attention again. However, it is also known that the mechanism is annoyed by two serious problems, that is, a domain wall problem and goodness of global symmetry. Any global symmetry is considered not to be exact due to the quantum effect of gravity. In this paper, we consider a solution to these problems, in which quark mass hierarchy and mixing, neutrino mass generation and existence of dark matter are closely related. In our solution, PQ symmetry is assumed to be induced through symmetry breaking at an intermediate scale of a local U(1) symmetry, and a global U(1) symmetry which plays a role of Froggatt-Nielsen symmetry . In the lepton sector, a remnant of the PQ symmetry controls neutrino mass generation and dark matter existence.Comment: 25 pages, 2 figures, references added, figures correctly include