arXiv subjects

T. Yanagida

Publications and source records attributed to T. Yanagida.

At least 55 records · Page 3Linked to original sources

arXiv2001

Leptogenesis via $LH_{u}$ flat direction with a gauged $U(1)_{B-L}$

We study the supersymmetric leptogenesis via $LH_{u}$ flat direction with a gauged $U(1)_{B-L}$ symmetry. We find that the resultant baryon asymmetry is enhanced compared with the case without a gauged $U(1)_{B-L}$ symmetry. The baryon asymmetry is proportional to the reheating temperature of inflation, but it is independent of the gravitino mass. If high reheating temperatures of inflation $T_{R}\sim 10^{10}\GEV$ are available, the mass of the lightest neutrino, $m_{ν1}\sim 10^{-4}\EV$, is small enough to explain the baryon asymmetry in the present universe. Furthermore, the gravitino mass independence of the produced baryon asymmetry allows us to explain the present baryon asymmetry even in low energy SUSY breaking scenarios such as gauge-mediation models. Our leptogenesis scenario is also very special in the sense that it is completely free from the serious Q-ball formation problem.

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arXiv2001

Leptogenesis from $\widetilde{N}$-dominated early universe

We investigate in detail the leptogenesis by the decay of coherent right-handed sneutrino $\widetilde{N}$ having dominated the energy density of the early universe, which was originally proposed by HM and TY. Once the $\widetilde{N}$ dominant universe is realized, the amount of the generated lepton asymmetry (and hence baryon asymmetry) is determined only by the properties of the right-handed neutrino, regardless of the history before it dominates the universe. Moreover, thanks to the entropy production by the decay of the right-handed sneutrino, thermally produced relics are sufficiently diluted. In particular, the cosmological gravitino problem can be avoided even when the reheating temperature of the inflation is higher than $10^{10}\GeV$, in a wide range of the gravitino mass $m_{3/2}\simeq 10\MeV$--$100\TeV$. If the gravitino mass is in the range $m_{3/2}\simeq 10\MeV$--$1\GeV$ as in the some gauge-mediated supersymmetry breaking models, the dark matter in our universe can be dominantly composed of the gravitino. Quantum fluctuation of the $\widetilde{N}$ during inflation causes an isocurvature fluctuation which may be detectable in the future.

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arXiv2001

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.

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arXiv2001

Semi-Simple Unification on T^6/Z_12 Orientifold in the Type IIB Supergravity

The semi-simple unification model based on SU(5)_GUT \times U(3)_H gauge group is an interesting extension of the minimal SU(5)_GUT grand unification theory (GUT), since it solves the two serious problems in the standard GUT: the triplet-doublet splitting problem and the presence of dangerous dimension five operators for proton decay. Here, the extra U(3)_H gauge interaction plays a crucial role on the GUT breaking. In this paper, we show that the full multiplet structure of the U(3)_H sector required for the desired GUT breaking is reproduced naturally on T^6/Z_12 orientifold in the type IIB supergravity with a D3-D7 system. The SU(5)_GUT vector multiplet lives on D7-branes and the U(3)_H sector resides on D3-branes. We also show that various interesting features in the original SU(5)_GUT \times U(3)_H model are explained in the present brane-world scenario. A possible extension to the type IIB string theory is also discussed.

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arXiv2001

Nonanomalous R-symmetry in supersymmetric unified theories of quarks and leptons

A discrete R-symmetry often appears as an exact gauge symmetry in the low energy effective theory of superstring theories. We search for such discrete R-symmetries from a phenomenological point of view and find that Z_{9R} and Z_{18R} are candidates of the nonanomalous R-symmetry in the case of the minimal supersymmetric standard model. We also find Z_{4R} and Z_{20R} in the case that quarks and leptons are embedded in the SU(5) GUT multiplets. Interesting is that in the latter case all the solutions predict some extra matter multiplets and we find that the simplest choice is to take a pair of {\bf 5} and {\bf 5}^* of SU(5)_{GUT} whose mass is of order the SUSY breaking scale \sim 1 TeV. We emphasize that the presence of such extra matters is testable in future collider experiments.

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arXiv2001

Affleck-Dine baryo/leptogenesis with a gauged U(1)(B-L)

We briefly review the present status of Affleck-Dine baryo/leptogenesis scenarios in the minimal supersymmetric standard model (MSSM) in the context of the gravity-mediated SUSY breaking, and show that there is a serious cosmological problem in the Affleck-Dine mechanism. That is, the late decay of the associated large Q-balls leads to the over production of the lightest supersymmetric particles. Then, we point out that the minimal extension of the MSSM by introducing a gauged $U(1)_{B-L}$ symmetry naturally solves this problem. Here, the breaking scale of the $U(1)_{B-L}$ can be determined quite independently of the reheating temperature from the required baryon asymmetry. It is extremely interesting that the obtained scale of the $U(1)_{B-L}$ breaking is well consistent with the one suggested from the seesaw mechanism to explain the recent neutrino-oscillation experiments. We consider that the present scenario provides a new determination of the $U(1)_{B-L}$ breaking scale fully independent of the neutrino masses. We also comment on viability of the present scenario in anomaly-mediated SUSY breaking models.

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arXiv2001

Semi-simple group unification in the supersymmetric brane world

The conventional supersymmetric grand unified theories suffer from two serious problems, the large mass splitting between doublet and triplet Higgs multiplets, and the too long lifetime of the proton. A unification model based on a semi-simple group SU(5)_{GUT} \times U(3)_H has been proposed to solve both of the problems simultaneously. Although the proposed model is perfectly consistent with observations, there are various mysteries. In this paper, we show that such mysterious features in the original model are naturally explained by embedding the model into the brane world in a higher dimensional space-time. In particular, the relatively small gauge coupling constant of the SU(5)_{GUT} at the unification energy scale is a consequence of relatively large volume of extra dimensions. Here, we put the SU(5)_{GUT} gauge multiplet in a 6-dimensional bulk and assume all fields in the U(3)_H sector to reside on a 3-dimensional brane located in the bulk. On the other hand, all chiral multiplets of quarks, leptons and Higgs are assumed to reside on a 3-brane at a T^2/Z_4 orbifold fixed point. The quasi-N=2 supersymmetry in the hypercolor U(3)_H sector is understood as a low-energy remnant of the N=4 supersymmetry in a 6-dimensional space-time. We further extend the 6-dimensional model to a 10-dimensional theory. Possible frameworks of string theories are also investigated to accommodate the present brane-world model. We find that the type IIB string theory with D3-D7 brane structure is an interesting candidate.

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arXiv2001

Natural Chaotic Inflation in Supergravity and Leptogenesis

We comprehensively investigate a chaotic inflation model proposed recently in the framework of supergravity. In this model, the form of Kähler potential is determined by a symmetry, that is, the Nambu-Goldstone-like shift symmetry, which guarantees the absence of the exponential factor in the potential for the inflaton field. Though we need the introduction of small parameters, the smallness of the parameters is justified also by symmetries. That is, the zero limit of the small parameters recovers symmetries, which is natural in the 't Hooft's sense. The leptogenesis scenario via the inflaton decay in this chaotic inflation model is also discussed. We find that the lepton asymmetry enough to explain the present baryon number density is produced for low reheating temperatures avoiding the overproduction of gravitinos.

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arXiv2001

Reheating-temperature independence of cosmological baryon asymmetry in Affleck-Dine leptogenesis

In this paper we point out that the cosmological baryon asymmetry in our universe is generated almost independently of the reheating temperature $T_R$ in Affleck-Dine leptogenesis and it is determined mainly by the mass of the lightest neutrino, $m_{ν_1}$, in a wide range of the reheating temperature $T_R\simeq 10^5$--$10^{12}$ GeV. The present baryon asymmetry predicts the $m_{ν_1}$ in a narrow region, $m_{ν_1}\simeq (0.3$--$1)\times 10^{-9}$ eV. Such a small mass of the lightest neutrino leads to a high predictability on the mass parameter $m_{ν_e ν_e}$ contributing to the neutrinoless double beta decay. We also propose an explicit model in which such an ultralight neutrino can be naturally obtained.

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arXiv2000

Variations on Supersymmetry Breaking and Neutrino Spectra

The problem of generating light neutrinos within supersymmetric models is discussed. It is shown that the hierarchy of scales induced by supersymmetry breaking can give rise to suppression factors of the correct order of magnitude to produce experimentally allowed neutrino spectra.

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arXiv2000

N = 2 Supersymmetry in a Hybrid Inflation Model

The slow roll inflation requires an extremely flat inflaton potential. The supersymmetry (SUSY) is not only motivated from the gauge hierarchy problem, but also from stabilizing that flatness of the inflaton potential against radiative corrections. However, it has been known that the Planck suppressed higher order terms in the Kähler potential receive large radiative corrections loosing the required flatness in the N = 1 supergravity. We propose to impose a global N = 2 SUSY on the inflaton sector. What we find is that the N = 2 SUSY Abelian gauge theory is exactly the same as the desired hybrid inflation model. The flat potential at the tree level is not our choice of parameters but a result of the symmetry. We further introduce a cut-off scale of the theory which is lower than the Planck scale. This lower cut-off scale suppresses the supergravity loop corrections to the flat inflaton potential.

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arXiv2000

Vacuum Instability in Anomaly Mediation Models with Massive Neutrinos

We study the vacuum stability in the anomaly mediated supersymmetry (SUSY) breaking models with massive neutrinos. It is shown that, because of the seesaw-induced mass terms for neutrinos, the true vacuum has a large negative cosmological constant provided that the vacuum where we now live has an (almost) vanishing cosmological constant. Although the quantum transition into the true vacuum from our false vacuum is highly suppressed, the thermal transition at high temperatures may not be neglected because of the thermal excitations. However, we find that the thermal transition is, in fact, negligibly small and hence the anomaly mediation models are cosmologically safe. Thus, we conclude that the reheating temperature T_R could be very high (e.g. T_R >> 10^10 GeV) in the anomaly mediation models even with the seesaw-induced mass terms for neutrinos.

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arXiv2000

LSND, SN1987A, and CPT Violation

We point out that neutrino events observed at Kamiokande and IMB from SN1987A disfavor the neutrino oscillation parameters preferred by the LSND experiment. For $Δm^2 > 0$ (the light side), the electron neutrinos from the neutronization burst would be lost, while the first event at Kamiokande is quite likely to be due to an electron neutrino. For $Δm^2 < 0$ (the dark side), the average energy of the dominantly $\barν_e$ events is already lower than the theoretical expectations, which would get aggravated by a complete conversion from $\barν_μ$ to $\barν_e$. If taken seriously, the LSND data are disfavored independent of the existence of a sterile neutrino. A possible remedy is CPT violation, which allows different mass spectra for neutrinos and anti-neutrinos and hence can accommodate atmospheric, solar and LSND data without a sterile neutrino. If this is the case, Mini-BooNE must run in $\barν$ rather than the planned $ν$ mode to test the LSND signal. We speculate on a possible origin of CPT violation.

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arXiv2000

Supersymmetric seesaw model for the (1+3)-scheme of neutrino masses

A four-neutrino spectrum with a sterile neutrino without significant involvement in the atmospheric and solar neutrino oscillation experiments has been recently advocated as the correct picture to explain all existing experimental data. We propose a supersymmetric model in which this picture can be naturally implemented. In this model, the mass for the mainly active neutrino eigenstates is induced by the seesaw mechanism with a large intermediate scale $M_R$, whereas the mass for the mainly sterile neutrino state is closely related to supersymmetry breaking.

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arXiv2000

A Constraint on Yukawa-Coupling Unification from Lepton-Flavor Violating Processes

We present a new constraint on a lepton mixing matrix $V$ from lepton-flavor violating (LFV) processes in supersymmetric standard models with massive neutrinos. Here, we assume Yukawa-coupling unification $f_{ν3}\simeq f_{\rm top}$, in which $τ$-neutrino Yukawa coupling $f_{ν3}$ is unified into top-quark Yukawa coupling $f_{\rm top}$ at the unification scale $M_*\simeq 3\times 10^{16}$ GeV. We show that the present experimental bound on $μ\to e γ$ decay already gives a stringent limit on the lepton mixing (typically $V_{13}<0.02$ for $V_{23}=1/\sqrt{2}$). Therefore, many existing neutrino-mass models are strongly constrained. Future improvement of bounds on LFV processes will provide a more significant impact on the models with the Yukawa-coupling unification. We also stress that a precise measurement of a neutrino mixing $(V_{MNS})_{e3}$ in future neutrino experiments would be very important, since the observation of non-zero $(V_{MNS})_{e3}$, together with negative experimental results for the LFV processes, have a robust potential to exclude a large class of SUSY standard models with the Yukawa-coupling unification.

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arXiv2000

Topological Inflation in Supergravity

We investigate a topological inflation model in supergravity. By means of numerical simulations, it is confirmed that topological inflation can take place in supergravity. We also show that the condition for successful inflation depends not only on the vacuum-expectation value (VEV) of inflaton field but also on the form of its Kähler potential. In fact, it is found that the required VEV of the inflaton $ϕ$ can be as small as $<ϕ> \simeq 1 \times M_G$, where $M_{G}$ is the gravitational scale.

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arXiv2000

Solving the Gravitino Problem by Axino

In a large class of supersymmetric (SUSY) axion model the mass of axino $\axino$ (a fermionic superpartner of the axion) is predicted as $m_{\axino} \lesssim {\cal O}(1)$ keV. Thus, the axino is the lightest SUSY particle (LSP). We pointed out that such a light axino provides a natural solution to the gravitino problem, if the gravitino is the next LSP. We derive a constraint on the reheating temperature $T_R$ of inflation, $T_R \lesssim 10^{15}$ GeV for the gravitino mass $m_{3/2} \simeq 100$ GeV, which is much weaker than that obtained in the minimal SUSY standard model.

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arXiv2000

Low-Energy Predictions of Lopsided Family Charges

We consider the Froggatt-Nielsen (FN) mechanism reproducing the observed mass hierarchies and mixing angles for quarks and leptons. The large $ν_μ$-$ ν_τ$ mixing suggested from recent Superkamiokande experiments on the atmospheric neutrinos implies lopsided FN U(1) charges for the lepton doublets. There are two possible charge assignments to generate the large $ν_μ$-$ ν_τ$ mixing. We point out that the two models with different charge assignments have distinct low-energy predictions and hence they are distinguishable in future neutrino experiments on such as CP violation and $2β0ν$ decay.

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