arXiv subjects

T. Yanagida

Publications and source records attributed to T. Yanagida.

At least 109 records · Page 6Linked to original sources

arXiv1998

Atmospheric Neutrino Oscillation and a Phenomenological Lepton Mass Matrix

We propose simple phenomenological lepton mass matrices which describe the three neutrinos almost degenerate in mass, leading to a very large mixing angle between $ν_μ$ and $ν_τ$, as consistent with a recent report on atmospheric neutrino oscillation from the Superkamiokande collaboration. Our matrix model also gives $ν_e-ν_μ$ mixing in agreement with the value required for neutrino oscillation to explain the solar neutrino problem.

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arXiv1998

A Gauge Mediation Model of Dynamical Supersymmetry Breaking without Color Instability

We construct a gauge mediation model of dynamical supersymmetry breaking (DSB) based on a vector-like gauge theory, in which there is a unique color-preserving true vacuum. The DSB scale $Λ/4π$ turns out to be as high as $Λ/4π\simeq 10^{8-9} GeV$, since the transmission of the DSB effects to the standard model sector is completed through much higher loops. This model is perfectly natural and phenomenologically consistent. We also stress that the dangerous D-term problem for the messenger U(1)_m is automatically solved by a charge conjugation symmetry in the vector-like gauge theory.

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arXiv1997

Atmospheric Neutrino Oscillation and Large Lepton-Flavour Violation in the SUSY SU(5) GUT

The atmospheric neutrino anomaly reported by the super-Kamiokande collaboration suggests existence of a large flavour violating Yukawa coupling in the lepton sector. We discuss lepton-flavour violation at low energies in the framework of the supersymmetric SU(5)_{GUT} model with right-handed neutrinos. We find that for a wide range of parameter space suggested from the atmospheric neutrino anomaly, if the tau-neutrino Yukawa coupling is as large as that of top quark, lower bounds of the branching ratios of mu -> e gamma, tau -> mu gamma, and the mu-e conversion rate on $^{48}_{22}Ti$ become around 10^{-14}, 10^{-9}, and 10^{-16}, respectively. These reaction rates may be in the region accessible by near future experiments.

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arXiv1997

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

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, $\sin^22θ_{ν_μν_τ} \simeq 1$, 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.

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arXiv1997

Primordial Black Hole Formation in a Double Inflation Model in Supergravity

It has been recently pointed out that the initial value problem in new inflation models is naturally solved by supergravity effects if there exists a pre-inflation before the new inflation. We study this double inflation model in details and find that density fluctuations on small cosmological scales are much larger than those on large scales due to peculiar property of the new inflation. We show that this results in production of primordial black holes which have $\sim 1 M_{\odot}$ masses in a certain parameter region of the double inflation model. We stress that these black holes may be identified with MACHOs observed in the halo of our galaxy.

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arXiv1997

Dimension-six Proton Decays in the Modified Missing Doublet SU(5) Model

Dimension-five operators for nucleon decays are suppressed in the modified missing doublet (MMD) model in the supersymmetric SU(5) grand unification. We show that nonrenormalizable interactions decrease the unification scale in the MMD model which increases the nucleon decay rate of dimension-six operators by a significant amount. We find that the theoretical lower bound on the proton life time $τ(p \to \e^+ π^0)$ is within the observable range at SuperKamiokande.

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arXiv1997

Cosmological Constraint on the String Dilaton in Gauge-mediated Supersymmetry Breaking Theories

The dilaton field in string theories (if exists) is expected to have a mass of the order of the gravitino mass $m_{3/2}$ which is in a range of $10^{-2}$keV--1GeV in gauge-mediated supersymmetry breaking models. If it is the case, the cosmic energy density of coherent dilaton oscillation easily exceeds the critical density of the present universe. We show that even if this problem is solved by a late-time entropy production (thermal inflation) a stringent constraint on the energy density of the dilaton oscillation is derived from experimental upperbounds on the cosmic X($γ$)-ray backgrounds. This excludes an interesting mass region, $500keV \lesssim m_{3/2} \lesssim 1GeV$, in gauge-mediated supersymmetry breaking models.

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arXiv1997

Domain Wall Problem of Axion and Isocurvature Fluctuations in Chaotic Inflation Models

We study the domain wall problem of an axion in chaotic inflation models. We show that the production of domain walls does not occurs if the Peccei-Quinn scalar has a flat potential and its breaking $F_{a}$ is larger than $\sim 10^{15}$GeV. We find that too large isocurvature fluctuations are produced for such high $F_{a}$. In order for those isocurvature fluctuations to be consistent with observations of the large scale structure of the universe, the self-coupling constant $g$ and the breaking scale of the Peccei-Quinn scalar should be $g \simeq (1-2.8)\times 10^{-13}$ and $F_{a} \simeq (0.6-1.5)\times 10^{15}$GeV, respectively. In particular the value of self-coupling constant is almost the same as that required for a chaotic inflation, which strongly suggests that the Peccei-Quinn scalar itself is an inflaton.

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arXiv1997

Dynamical Tuning of the Initial Condition for New Inflation in Supergravity

We point out that for a class of `new inflation' models in supergravity the required initial value of the inflaton field is dynamically set if there is another inflation (`pre-inflation') before the `new inflation'. We study the dynamics of both inflatons by taking a hybrid inflation model for the `pre-inflation' as an example. We find out that our `new inflation' model provides reheating temperatures T_R \simeq 10 MeV - 10^4 GeV low enough to avoid the gravitino problem even in gauge-mediated supersymmetry-breaking models. We also construct a model where the scale for the `new inflation' is generated by nonperturbative dynamics of a supersymmetric gauge theory.

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arXiv1997

Cosmological Axion Problem in Chaotic Inflationary Universe

We investigate two cosmological axion problems (isocurvature fluctuations and domain-wall formation) in chaotic inflationary universe. It is believed that these problems are solved if potential for the Peccei-Quinn scalar field is very flat. However, we find that too many domain walls are produced through parametric resonance decay of the Peccei-Quinn scalar field. Only the axion model with N=1(N: QCD anomaly factor) is consistent with observations. We also point out that the flat potential is naturally obtained in a supersymmetric extension of the Peccei-Quinn model. If Peccei-Quinn breaking scale $F_a$ is about 10^{12} GeV, this model predicts anisotropies of cosmic microwave background radiation due to the axion isocurvature fluctuations which may be detectable in future observations.

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arXiv1997

Direct-Transmission Models of Dynamical Supersymmetry Breaking

We systematically construct gauge-mediated supersymmetry(SUSY)-breaking models with direct transmission of SUSY-breaking effects to the standard-model sector. We obtain a natural model with the gravitino mass $m_{3/2}$ smaller than 1 keV as required from the standard cosmology. If all Yukawa coupling constants are of order one,the SUSY-breaking scale $m_{SUSY}$ transmitted into the standard-model sector is given by $m_{SUSY} \simeq 0.1 α_i/(4π) Λ$ where $Λ$ is the original dynamical SUSY-breaking scale. Imposing $m_{SUSY} \simeq (10^2-10^3)$ GeV, we get $Λ\simeq (10^5-10^6)$ GeV, which yields the gravitino mass $m_{3/2}\simeq (10^{-2}-1)$ keV.

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arXiv1997

R-invariant Natural Unification

We construct R-invariant unification models where a pair of massless Higgs doublets is naturally obtained. The masslessness of the Higgs doublets is guaranteed by the unbroken R symmetry. Mass generation for the Higgs doublets is considered from various viewpoints.

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arXiv1997

Are Isocurvature Fluctuations of the M-theory Axion Observable?

Banks and Dine have recently shown that the M theory naturally accommodates the Peccei-Quinn axion. Since the decay constant $F_a$ of the axion is large as $F_a \simeq 10^{15}-10^{16}$GeV, the halo axion is hardly detected in coming axion-search experiments. However, we show that isocurvature fluctuations of the M-theory axion produced at the inflationary epoch are most likely detectable in future satellite experiments on anisotropies of the cosmic microwave background radiation.

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arXiv1997

A Solution to the $μ$ Problem in Gauge-mediated Supersymmetry-breaking Models

We point out that a sector required to set the cosmological constant to zero in gauge-mediated supersymmetry-breaking models naturally produces a supersymmetry-invariant mass ($μ$ term) for Higgs doublets of the order of the electroweak scale. Since this new sector preserves the supersymmetry, it does not generate supersymmetry-breaking masses for the Higgs doublets and thus the $μ$ problem is solved.

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arXiv1997

Constraint on Cosmic Density of the String Moduli Field in Gauge-Mediated Supersymmetry-Breaking Theories

We derive a constraint on the cosmic density of string moduli fields in gauge-mediated supersymmetry-breaking theories by requiring that photons emitted from the unstable moduli fields should not exceed the observed X-ray backgrounds. Since mass of the moduli field lies in the range between $O(0.1)$keV and $O(1)$MeV and the decay occurs through a gravitational interaction, the lifetime of the moduli field is much longer than the age of the present universe. The obtained upperbound on their cosmic density becomes more stringent than that from the unclosure condition for the present universe for the mass greater than about 100keV.

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arXiv1996

Natural New Inflation in Broken Supergravity

We consider a natural new inflationary model in broken supergravity based on an R symmetry. The model predicts a concrete relation between the amplitude of primordial density fluctuations and the scale of supersymmetry breaking. The observed value of the density fluctuations is obtained for the gravitino mass of order the weak scale along with a power-law spectral index considerably less than one, which may be tested in future observations.

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arXiv1996

Affleck-Dine Baryogenesis after Thermal Inflation

We argue that an extension of the Minimal Supersymmetric Standard Model that gives rise to viable thermal inflation, and so does not suffer from a Polonyi/moduli problem, should contain right-handed neutrinos which acquire their masses due to the vacuum expectation value of the flaton that drives thermal inflation. This strongly disfavours SO(10) Grand Unified Theories. The $μ$-term of the MSSM should also arise due to the vev of the flaton. With the extra assumption that $ m_L^2 - m_{H_u}^2 < 0 $, but of course $ m_L^2 - m_{H_u}^2 + |μ|^2 > 0 $, we show that a complicated Affleck-Dine type of baryogenesis employing an $LH_u$ $D$-flat direction can naturally generate the baryon asymmetry of the Universe.

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arXiv1996

Is the ee gamma gamma+ Missing E(T) Event an Evidence of the Light Axino?

We point out that if the Peccei-Quinn symmetry breaking scale $F_a$ is in a range of the hadronic axion window ($F_a\sim 10^6$GeV), the ee$γγ+/\!\!\!\! E_T$ event in the CDF experiment can be naturally explained by a no-scale supergravity model with a light axino. We also stress that the hadronic axion window still survives the intergalactic photon search, since a large entropy production due to the decay of Polonyi field yields a substantial dilution of the cosmic axion density.

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