arXiv subjects
Publications and source records attributed to T. Asaka.
We study the initial value problem of new inflation models in the scheme of supergravity. We find that an inflaton generally acquires a mass of the order of the Hubble parameter via gravitationally suppressed interactions with particles in the thermal bath of the universe. This additional mass term stabilizes dynamically the inflaton at the local maximum of the potential, which offers the initial condition of new 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.
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.
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.
In superstring theories, there exist various dilaton and modulus fields which masses are expected to be of the order of the gravitino mass $m_{3/2}$. These fields lead to serious cosmological difficulties, so called ``cosmological moduli problem'', because a large number of moduli particles are produced as the coherent oscillations after the primordial inflation. We make a comprehensive study whether the thermal inflation can solve the cosmological moduli problem in the whole modulus mass region $m_ϕ\sim 10 eV - 10^4 GeV$ predicted by both hidden sector supersymmetry (SUSY) breaking and gauge-mediated SUSY breaking models. In particular, we take into account the primordial inflation model whose reheating temperature is so low that its reheating process finishes after the thermal inflation ends. We find that the above mass region $m_ϕ(\simeq m_{3/2}) \sim 10 eV - 10^4 GeV$ survives from various cosmological constraints in the presence of the thermal inflation.
We investigate cosmological moduli problem in the gauge-mediated supersymmetry breaking (GMSB). A mini-inflation (oscillating inflation) takes place when a scalar field corresponding to the flat direction in GMSB oscillates along the logarithmic potential induced by the gauge-mediation mechanism. It is shown that this oscillating inflation can sufficiently dilute the relic abundance of the string moduli for some ranges of parameters in the GMSB models.
Recently we have proposed a simple hadronic axion model within gauge-mediated supersymmetry breaking. In this paper we discuss various cosmological consequences of the model in great detail. A particular attention is paid to a saxion, a scalar partner of an axion, which is produced as a coherent oscillation in the early universe. We show that our model is cosmologically viable, if the reheating temperature of inflation is sufficiently low. We also discuss the late decay of the saxion which gives a preferable power spectrum of the density fluctuation in the standard cold dark matter model when compared with the observation.
A simple hadronic axion model is proposed in the framework of gauge-mediated supersymmetry breaking. Dynamics of Peccei-Quinn symmetry breaking is governed by supersymmetry breaking effects and the Peccei-Quinn breaking scale $f_{PQ}$ is inversely proportional to the gravitino mass. The gravitino mass range which corresponds to the axion window $f_{PQ} \simeq 10^{9}$ GeV -- $10^{13}$ GeV lies in the region predicted by gauge-mediated supersymmetry breaking models. The model is also shown to be cosmologically viable.
We examine the $X$-ray spectrum from the decay of the dark-matter moduli with mass $\sim {\cal O}(100)$keV, in particular, paying attention to the line spectrum from the moduli trapped in the halo of our galaxy. It is found that with the energy resolution of the current experiments ($\sim 10$%) the line intensity is about twice stronger than that of the continuum spectrum from the moduli that spread in the whole universe. Therefore, in the future experiments with higher energy resolutions it may be possible to detect such line photons. We also investigate the $γ$-ray spectrum emitted from the decay of the multi-GeV moduli. It is shown that the emitted photons may form MeV-bump in the $γ$-ray spectrum. We also find that if the modulus mass is of the order of 10 GeV, the emitted photons at the peak of the continuum spectrum loses their energy by the scattering and the shape of the spectrum is significantly changed, which makes the constraint weaker than that obtained in the previous works.
A generic class of string theories predicts the existence of light moduli fields, and they are expected to have masses $m_ϕ$ comparable to the gravitino mass $m_{3/2}$ which is in a range of $10^{-2}$keV--1GeV in gauge-mediated supersymmetry breaking theories. Such light fields with weak interactions suppressed by the Planck scale can not avoid some stringent cosmological constraints, that is, they suffer from `cosmological moduli problems'. We show that all the gravitino mass region $10^{-2}$keV $\lesssim m_{3/2} \lesssim$ 1GeV is excluded by the constraints even if we incorporate a late-time mini-inflation (thermal inflation). However, a modification of the original thermal inflation model enables the region $10^{-2}$keV $\lesssim m_{3/2} \lesssim$ 500keV to survive the constraints. It is also stressed that the moduli can be dark matter in our universe for the mass region $10^{-2}$keV $\lesssim m_ϕ\lesssim$ 100keV.
We consider a scenario where the top-quark mass is generated dynamically, and study the implication of the present experimental values for $m_t$ and the $T$ parameter. We assume technicolor-like scenario for inducing the $W$ mass and an effective four fermi operator for inducing the top-quark mass. We also assume that only this four fermi operator is relevant at low energy. Then we estimate in detail the strength $G$ and the intrinsic mass scale $M$ of the four fermi operator. Unitarity bound is used to quantify the strength of $G$. We find that $G/4π\sim {\cal O}(1)$ and that $M$ is of the order of $Λ_{TC} \simeq 1 \sim 2$~TeV or less. Namely the four fermi operator cannot be treated as `point-like' around the electroweak scale. Furthermore we estimate the contribution of the four fermi operator to the $T$ parameter. We find that the QCD correction to the top-quark mass function reduces the contribution to the $T$ parameter by about 40\%. By comparing the results with the present experimental bound, we obtain another upper bound on $M$ which is typically in several to 10~TeV region.
We report first results from the investigation on probing dynamical mechanism of the electroweak symmetry breaking using top quark. We consider the case where the top mass originates from a fermion-antifermion pair condensate, which necessitates (i)strong interaction to cause condensation, and (ii)4-fermi interaction to give top mass. From the observed top mass and the unitarity constraint, we obtain, for the 4-fermi interaction (ii), lower bounds for its strength and upper bounds for its intrinsic new mass scale as we vary the type of the strong interaction (i). Bethe-Salpeter and Schwinger-Dyson equations are solved numerically to study the dynamical symmetry breaking effect semi-quantitatively.
We calculate four-fermion decays of a Higgs boson via $WW$ and/or $ZZ$ intermediate states for Higgs masses below $m_W$. We examine models with a doubly-charged Higgs boson $H^{++}$ and show that the four-fermion decay is the dominant mode for a wide range of parameter space. Existing searches for $H^{++}$ in $Z$ decays have not looked for this mode. We also derive four-fermion decay rate for a neutral Higgs boson.