arXiv subjects

T. Yanagida

Publications and source records attributed to T. Yanagida.

At least 73 records · Page 4Linked to original sources

arXiv2000

Natural Chaotic Inflation in Supergravity

We propose a chaotic inflation model in supergravity. In the model the Kähler potential has a Nambu-Goldstone-like shift symmetry of the inflaton chiral multiplet which ensures the flatness of the inflaton potential beyond the Planck scale. We show that the chaotic inflation naturally takes place by introducing a small breaking term of the shift symmetry in the superpotential. This may open a new branch of model building for inflationary universe in the framework of supergravity.

hep-ph↗

arXiv2000

Natural Mass Generation for the Sterile Neutrino

We point out that there is a serious cosmological problem in the supersymmetric standard model if a sterile neutrino is responsible for the solar neutrino oscillation, and propose a possible solution to this problem. We show that our solution induces naturally a mass of order 10^{-4} eV for the sterile neutrino, which is deeply related to the mechanism of supersymmetry breaking.

hep-ph↗

arXiv2000

Affleck-Dine Leptogenesis with an Ultralight Neutrino

We perform a detailed analysis on Affleck-Dine leptogenesis taking into account the thermal effects on the dynamics of the flat direction field $ϕ$. We find that an extremely small mass for the lightest neutrino $ν_1$, ${m_ν}_1\lsim 10^{-8}$ eV, is required to produce enough lepton-number asymmetry to explain the baryon asymmetry in the present universe. We impose here the reheating temperature after inflation $T_R$ to be $T_R\lsim 10^8$ GeV to solve the cosmological gravitino problem. The required value of neutrino mass seems to be very unlikely the case since the recent Superkamiokande experiments suggest the masses of heavier two neutrinos $ν_2$ and $ν_3$ to be in a range of $10^{-1}$--$10^{-3}$ eV. We also propose a model to avoid this difficulty based on the Peccei-Quinn symmetry, where the required neutrino mass can be as large as ${m_ν}_1\simeq 10^{-4}$ eV.

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arXiv2000

Double Inflation in Supergravity and the Boomerang Observations

One of the biggest mystery of the recent observation of cosmic microwave background anisotropies by the boomerang team is insignificance of the second acoustic peak in the angular power spectrum. It is very difficult to explain such a low amplitude without assuming the higher baryon density than predicted by the Big Bang Nucleosynthesis (BBN). Employing the double inflation model in supergravity, we show that the density fluctuations produced by this inflation model can produce a sufficient low second acoustic peak with the standard value of the baryon density from BBN. It is shown that these density fluctuations are also consistent with the observations of cluster abundances and galaxy distributions.

astro-ph↗

arXiv2000

Double inflation in supergravity and the primordial black hole formation

We study a double inflation model (a hybrid inflation + a new inflation) in supergravity and discuss the formation of primordial black holes (PBHs) with mass \sim 10^{-20}-10^{5}M_{\odot}. We find that in a wide range of parameter space, we obtain PBHs which amount to Ω\simeq 1, i.e., PBH dark matter. Also, we find a set of inflation parameters which produces PBHs evaporating now. Those PBHs may be responsible for antiproton fluxes observed by the BESS experiment.

hep-ph↗

arXiv2000

Bulk U(1) Messenger

We propose a new U(1) gauge interaction in the bulk in higher dimensional spacetime, which transmits supersymmetry-breaking effects on the hidden brane to the observable our brane. We find that rather small gauge coupling constant of U(1)_{bulk}, $α_{bulk} \simeq 5 \times 10^{-4}$, is required for a successful phenomenology. This result implies the compactification length $L$ of the extra dimension to be $L^{-1} \simeq 2 \times 10^{15}GeV$ for (4+1)-dimensional spacetime. This large compactification length $L$ is a crucial ingredient to suppress unwanted flavor-changing neutral currents and hence our proposal is very consistent with the Randall-Sundrum brane-world scenario.

hep-ph↗

arXiv2000

Double Inflation in Supergravity and the Large Scale Structure

The cosmological implication of a double inflation model with hybrid + new inflations in supergravity is studied. The hybrid inflation drives an inflaton for new inflation close to the origin through supergravity effects and new inflation naturally occurs. If the total e-fold number of new inflation is smaller than $\sim 60$, both inflations produce cosmologically relevant density fluctuations. Both cluster abundances and galaxy distributions provide strong constraints on the parameters in the double inflation model assuming $Ω_0=1$ standard cold dark matter scenario. The future satellite experiments to measure the angular power spectrum of the cosmic microwave background will make a precise determination of the model parameters possible.

hep-ph↗

arXiv2000

Quintessence Axion Potential Induced by Electroweak Instanton Effects

Recent cosmological observations suggest the presence of small but nonzero cosmological constant $Λ_{cos}^4 \simeq (2 \times 10^{-3} eV)^4$. It is an intriguing possibility that such a small cosmological constant is supplied by the potential energy density of an ultralight axion-like field (called as quintessence axion). If this axion couples to the electroweak gauge fields, its potential may be generated by the electroweak instanton effects. We calculate the axion potential assuming the supersymmetric standard model and obtain a surprising result that the induced energy density of the quintessence axion field is very close to the value suggested from the observations.

hep-ph↗

arXiv1999

Mass Generation for an Ultralight Axion

If a global chiral symmetry is explicitly broken by anomalies in nonabelian gauge theories, a pseudo Nambu-Goldstone boson (axion) associated with a spontaneous breakdown of such a global symmetry acquires a mass through nonperturbative instanton effects. We calculate the axion mass assuming a supersymmetric SU(2) gauge theory and show that the axion obtains an extremely small mass when the SU(2) gauge symmetry is broken down at very high energy, say at the Planck scale. We identify the axion with a hypothetical ultralight boson field proposed to account for a small but nonzero cosmological constant suggested from recent cosmological observations.

hep-ph↗

arXiv1999

Grand-Unification Scale Generation through the Anomalous U(1) Breaking

We discuss the anomalous U(1) gauge symmetry as a mechanism of generating the grand-unification scale. We conclude that unification to a simple group cannot be realized unless some parameters are ``tuned'', and that models with product gauge groups are preferred. We consider the ``R-invariant natural unification'' model with gauge groups SU(5)_{GUT} \times U(3)_H. In this model the doublet-triplet splitting problem is solved and the unwanted GUT relation m_s = m_μis avoided maintaining m_b = m_τ. Moreover, R-invariance suppresses the dangerous proton decays induced by dimension four and five operators.

hep-ph↗

arXiv1999

Superheavy Dark Matter and Thermal Inflation

The thermal inflation is the most plausible mechanism that solves the cosmological moduli problem naturally. We discuss relic abundance of superheavy particle $X$ in the presence of the thermal inflation assuming that its lifetime is longer than the age of the universe, and show that the long-lived particle $X$ of mass $10^{12}$--$10^{14}$ GeV may form a part of the dark matter in the present universe in a wide region of parameter space of the thermal inflation model. The superheavy dark matter of mass $\sim 10^{13}$ GeV may be interesting in particular, since its decay may account for the observed ultra high-energy cosmic rays if the lifetime of the $X$ particle is sufficiently long.

hep-ph↗

arXiv1999

Democratic Mass Matrices from Broken $O(3)_L\times O(3)_R$ Flavor Symmetry

We impose $O(3)_L\times O(3)_R$ flavor symmetry in the supersymmetric standard model. Three lepton doublets $\ell_i$ transform as an $O(3)_L$ triplet and three charged leptons $\bar e_i$ as an $O(3)_R$ triplet, while Higgs doublets $H$ and $\bar H$ are $O(3)_L\times O(3)_R$ singlets. We discuss a flavor $O(3)_L\times O(3)_R$ breaking mechanism that leads to "successful" phenomenological mass matrices, so-called "democratic" ones, in which the large $\n_μ-\n_τ$ mixing is naturally obtained. Three neutrinos have nearly degenerate masses of order $0.1\eV$ which may be accesible to future double $\b$-decay experiments. We extend our approach to the quark sector and show that it is well consistent with the observed quark mass hierarchies and the CKM matrix elements. However, the large mass of the top quark requires a relatively large coupling. constant.

hep-ph↗

arXiv1999

Long-Lived Superheavy Particles in Dynamical Supersymmetry-Breaking Models in Supergravity

Superheavy particles of masses $\simeq 10^{13}-10^{14} GeV$ with lifetimes $\simeq 10^{10}-10^{22} years$ are very interesting, since their decays may account for the ultra-high energy (UHE) cosmic rays discovered beyond the Greisen-Zatsepin-Kuzmin cut-off energy $E \sim 5 \times 10^{10} GeV$. We show that the presence of such long-lived superheavy particles is a generic prediction of QCD-like SU(N_c) gauge theories with N_f flavors of quarks and antiquarks and the large number of colors N_c. We construct explicit models based on supersymmetric SU(N_c) gauge theories and show that if the dynamical scale $Λ\simeq 10^{13}-10^{14} GeV$ and N_c = 6-10 the lightest composite baryons have the desired masses and lifetimes to explain the UHE cosmic rays. Interesting is that in these models the gaugino condensation necessarily occurs and hence these models may play a role of so-called hidden sector for supersymmetry breaking in supergravity.

hep-ph↗

arXiv1999

Cosmological Constants as Messenger between Branes

We present a supersymmetry-breaking scenario in which both the breaking in the hidden sector with no-scale type supergravity and that in the observable sector with gauge mediation are taken into account. The breaking scales in the hidden and observable sectors are related through the vanishing condition of the cosmological constant with a brane-world picture in mind. Suppressing flavor-changing neutral currents, we can naturally obtain the gravitino, Higgs(ino), and soft masses of the electroweak scale.

hep-ph↗

arXiv1999

Leptogenesis in Inflationary Universe

We investigate the leptogenesis via decays of heavy Majorana neutrinos which are produced non-thermally in inflaton decays. We make a comprehensive study on the leptogenesis assuming various supersymmetric (SUSY) models for hybrid, new and topological inflations. For an estimation of the lepton asymmetry we adopt the Froggatt-Nielsen mechanism for mass matrices of quarks and leptons. We find that all of these models are successful to produce the lepton asymmetry enough to explain the baryon number in the present universe. Here we impose low reheating temperatures such as $T_R \lesssim 10^8$ GeV in order to suppress the abundance of gravitinos not to conflict with the big-bang nucleosynthesis. Furthermore, we find that the leptogenesis works very well even with $T_R \simeq 10^{6}$ GeV in the SUSY hybrid or new inflation model. It is known that such a reheating temperature is low enough to suppress the abundance of gravitinos of mass $m_{3/2} \simeq 100$ GeV--1 TeV. Thus, the leptogenesis is fully consistent with the big-bang nucleosynthesis in a wide region of the gravitino mass.

hep-ph↗

arXiv1999

Observational Implications of Axionic Isocurvature Fluctuations

The axion is the most attractive candidate to solve the strong CP problem in QCD. If it exists, the inflationary universe produces axion fluctuations which are mixtures of isocurvature and adiabatic fluctuations in general. We investigate how large isocurvature fluctuations are allowed or favored in order to explain observations of the large scale structure of the present universe. Generic flat universe models with mixed (isocurvature + adiabatic) density fluctuations are studied. It is found that the observations are consistent with the mixed fluctuation model if the ratio αof the power spectrum of isocurvature fluctuations to that of adiabatic fluctuations is less than \sim 0.1. In particular, the mixed fluctuation model with α\sim 0.05, total matter density Ω_{0} =0.4, and Hubble parameter H_0=70 km/s/Mpc gives a very good fit to the observational data. Since the height of the acoustic peak in the angular power spectrum of the cosmic microwave background (CMB) radiation decreases significantly when the isocurvature fluctuations are present, the mixed fluctuation model can be tested in future satellite experiments. Ratios of the amplitude at the peak location to that at the COBE normalization scale for various models are given. Furthermore, we also obtain the amplitude of isocurvature fluctuations as a function of axion parameters and the Hubble parameter during the inflation. We discuss the axion fluctuations in some realistic inflation models and find that a significant amount of the isocurvature fluctuations are naturally produced.

astro-ph↗

arXiv1999

Leptogenesis in Inflaton Decay

We study a leptogenesis via decays of heavy Majorana neutrinos produced non-thermally in inflaton decays. We find that this scenario is fully consistent with existing supersymmetric inflation models such as for topological or for hybrid inflation and the Froggatt-Nielsen mechanism generating hierarchies in quark and lepton mass matrices. The reheating temperature $T_R$ of inflation may be taken as low as $T_R \simeq 10^8$ GeV to avoid the cosmological gravitino problem.

hep-ph↗

arXiv1999

Dynamical Inflation and Vacuum Selection

We consider an inflationary scenario where the energy scale of inflation stems from gauge theory dynamics. We point out its generic implications on vacuum selection of our universe, in particular, on determination of spacetime symmetries including its dimensions.

hep-ph↗