arXiv subjects

Takahiro Terada

Publications and source records attributed to Takahiro Terada.

At least 55 records · Page 3Linked to original sources

arXiv2018

Semianalytic Calculation of Gravitational Wave Spectrum Nonlinearly Induced from Primordial Curvature Perturbations

Whether or not the primordial gravitational wave (GW) produced during inflation is sufficiently strong to be observable, GWs are necessarily produced from the primordial curvature perturbations in the second order of perturbation. The induced GWs can be enhanced by curvature perturbations enhanced at small scales or by the presence of matter-dominated stages of the cosmological history. We analytically calculate the integral in the expression of the power spectrum of the induced GWs which is a universal part independent of the spectrum of the primordial curvature perturbations. This makes the subsequent numerical integrals significantly easy. In simple cases, we derive fully analytic formulas for the induced GW spectrum.

gr-qc↗

arXiv2017

Gravitino Problem in Inflation Driven by Inflaton-Polonyi Kähler Coupling

We discuss the cosmological gravitino problem in inflation models in which the inflaton potential is constructed from Kähler potential rather than superpotential: a representative model is $\overline{\text{D}3}$-induced geometric inflation. A critical ingredient in this type of models is the coupling of the inflaton and Polonyi (supersymmetry-breaking) field in the Kähler potential, which is needed to build the inflaton potential. We point out the same coupling let the inflaton dominantly decay into a pair of inflatino and gravitino causing the gravitino problem. We propose some possible solutions to this problem.

hep-ph↗

arXiv2017

Curvaton as dark matter with secondary inflation

We consider a novel cosmological scenario in which a curvaton is long-lived and plays the role of cold dark matter (CDM) in the presence of a short, secondary inflation. Non-trivial evolution of the large scale cosmological perturbation in the curvaton scenario can affect the duration of the short term inflation, resulting in the inhomogeneous end of inflation. Non-linear parameters of the curvature perturbation are predicted to be fNL ~ 5/4 and gNL ~ 0. The curvaton abundance can be well diluted by the short-term inflation and accordingly, it does not have to decay into the Standard Model particles. Then the curvaton can account for the present CDM with the isocurvature perturbation being sufficiently suppressed because both the adiabatic and CDM isocurvature perturbations have the same origin. As an explicit example, we consider the thermal inflation scenario and a string axion as a candidate for this curvaton-dark matter. We further discuss possibilities to identify the curvaton-dark matter with the QCD axion.

hep-ph↗

arXiv2017

Gravitino Problem in Minimal Supergravity Inflation

We study non-thermal gravitino production in the minimal supergravity inflation. In this minimal model utilizing orthogonal nilpotent superfields, the particle spectrum includes only graviton, gravitino, inflaton, and goldstino. We find that a substantial fraction of the cosmic energy density can be transferred to the longitudinal gravitino due to non-trivial change of its sound speed. This implies either a breakdown of the effective theory during reheating or a serious gravitino problem.

hep-ph↗

arXiv2016

Nonthermal Gravitino Production after Large Field Inflation

We revisit the nonthermal gravitino production at the (p)reheating stage after inflation. Particular attention is paid to large field inflation models with a $\mathbb{Z}_2$ symmetry, for which the previous perturbative analysis is inapplicable; and inflation models with a stabilizer superfield, which have not been studied non-perturbatively. It is found that in single-superfield inflation models (without the stabilizer field), nonthermal production of the transverse gravitino can be cosmologically problematic while the abundance of the longitudinal gravitino is small enough. In multi-superfield inflation models (with the stabilizer field), production of the transverse and longitudinal gravitinos is significantly suppressed, and they are cosmologically harmless. We also clarify the relation between the background field method used in the preheating context and the standard perturbative decay method to estimate the gravitino abundance.

hep-ph↗

arXiv2016

Reinterpretation of the Starobinsky model

The Starobinsky model of inflation, consistent with Planck 2015, has a peculiar form of the action, which contains the leading Einstein term $R$, the $R^2$ term with a huge coefficient, and negligible higher-order terms. We propose an explanation of this form based on compactification of extra dimensions. Once tuning of order $10^{-4}$ is accepted to suppress the linear term $R$, we no longer have to suppress higher-oder terms, which give nontrivial corrections to the Starobinsky model. We show our predictions of the spectral index, its runnings, and the tensor-to-scalar ratio. Finally, we discuss a possibility that quantum gravity may appear at the scale $Λ\gtrsim 5 \times 10^{15}$ GeV.

hep-th↗

arXiv2016

On SUSY Restoration in Single-Superfield Inflationary Models of Supergravity

We study the conditions of restoring supersymmetry (SUSY) after inflation in the supergravity-based cosmological models with a single chiral superfield and a quartic stabilization term in the Kähler potential. Some new, explicit, and viable inflationary models satisfying those conditions are found. The inflaton's scalar superpartner is dynamically stabilized during and after inflation. We also demonstrate a possibility of having small and adjustable SUSY breaking with a tiny cosmological constant.

hep-th↗

arXiv2016

Generalized Pole Inflation: Hilltop, Natural, and Chaotic Inflationary Attractors

A reformulation of inflationary model analyses appeared recently, in which inflationary observables are determined by the structure of a pole in the inflaton kinetic term rather than the shape of the inflaton potential. We comprehensively study this framework with an arbitrary order of the pole taking into account possible additional poles in the kinetic term or in the potential. Depending on the setup, the canonical potential becomes the form of hilltop or plateau models, variants of natural inflation, power-law inflation, or monomial/polynomial chaotic inflation. We demonstrate attractor behaviors of these models and compute corrections from the additional poles to the inflationary observables.

hep-th↗

arXiv2016

Structure of Kähler potential for D-term inflationary attractor models

Minimal chaotic models of D-term inflation predicts too large primordial tensor perturbations. Although it can be made consistent with observations utilizing higher order terms in the Kähler potential, expansion is not controlled in the absence of symmetries. We comprehensively study the conditions of Kähler potential for D-term plateau-type potentials and discuss its symmetry. They include the alpha-attractor model with a massive vector supermultiplet and its generalization leading to pole inflation of arbitrary order. We extend the models so that it can describe Coulomb phase, gauge anomaly is cancelled, and fields other than inflaton are stabilized during inflation. We also point out a generic issue for large-field D-term inflation that the masses of the non-inflaton fields tend to exceed the Planck scale.

hep-th↗

arXiv2016

Testing ATLAS Z+MET Excess with LHC Run 2

The ATLAS collaboration reported a 3sigma excess in the search of events containing on-Z dilepton, jets, and large missing momentum (MET) in the 8 TeV LHC run. Motivated by this excess, many models of new physics have been proposed. Recently, the ATLAS and CMS collaborations reported new results for similar Z+MET channels in the 13 TeV run. In this paper, we comprehensively discuss the consistency between the proposed models and the LHC results of Run 1 and Run 2. We find that in models with heavy gluino production, there is generically some tension between the 8 TeV and 13 TeV results. On the other hand, models with light squark production provide relatively better fitting to both results.

hep-ph↗

arXiv2015

Single-Superfield Helical-Phase Inflation

Large-field inflation in supergravity requires the approximate global symmetry needed to protect flatness of the scalar potential. In helical-phase inflation, the U(1) symmetry of the Kahler potential is assumed, the phase part of the complex scalar of a chiral superfield plays the role of inflaton, and the radial part is strongly stabilized. The original model of helical phase inflation, proposed by Li, Li and Nanopoulos (LLN), employs an extra (stabilizer) superfield. We propose a more economical new class of the helical phase inflationary models without a stabilizer superfield. As the specific examples, the quadratic, the natural, and the Starobinsky-type inflationary models are studied in our approach.

hep-th↗

arXiv2015

ATLAS Z Excess in Minimal Supersymmetric Standard Model

Recently the ATLAS collaboration reported a 3 sigma excess in the search for the events containing a dilepton pair from a Z boson and large missing transverse energy. Although the excess is not sufficiently significant yet, it is quite tempting to explain this excess by a well-motivated model beyond the standard model. In this paper we study a possibility of the minimal supersymmetric standard model (MSSM) for this excess. Especially, we focus on the MSSM spectrum where the sfermions are heavier than the gauginos and Higgsinos. We show that the excess can be explained by the reasonable MSSM mass spectrum.

hep-ph↗

arXiv2015

Inflation in Supergravity with a Single Superfield

Supergravity is a well-motivated theory beyond the standard model of particle physics, and a suitable arena to study high-energy physics at the early universe including inflation, whose observational evidences are growing more and more. Inflation in supergravity, however, cannot be easily described because of restrictions from supersymmetry (SUSY). The scalar potential has an exponential factor and a large negative term whereas a flat and positive potential is needed to realize inflation. The standard method to obtain such an inflationary scalar potential requires an additional superfield to the one containing inflaton. In this thesis, we propose and develop an alternative method which does not require the additional superfield and thus reduces the necessary degrees of freedom by half. That is, we study inflation in supergravity with only a single chiral superfield which contains inflaton. We accomplish it by introducing a higher dimensional term in the inflaton Kahler potential, which plays an important dual role: fixing the value of the scalar superpartner of the inflaton resulting in effective single field models, and ensuring the positivity of the inflaton potential at the large field region. Our proposal is not just particular models but rather a new framework to realize various inflationary models in supergravity. In particular, large field inflation in supergravity using one superfield without tuning has become possible for the first time. In our generic models, SUSY breaking at the inflationary scale by inflaton is not completely restored after inflation, so null results for SUSY search at the LHC are predicted for the simplest cases. Remarkably, however, it is possible with tuning to embed arbitrary positive semidefinite scalar potentials into supergravity preserving SUSY at the vacuum. Our discovery opens up an entirely new branch of model building of inflation in supergravity.

hep-th↗

arXiv2015

Toward Higgs inflation in the MSSM

Adopting a recently proposed single-superfield framework of supergravity inflation, we consider large field inflationary models in which MSSM Higgs-like fields play the role of the inflaton. In the simplest cases, the inflaton potential has a fractional power, which is different from that of the original Higgs inflation, and it can be tested by cosmological observations in near future. We find difficulties in identifying the inflaton with the MSSM Higgses and discuss possible candidates of the inflaton.

hep-ph↗

arXiv2015

Generic Scalar Potentials for Inflation in Supergravity with a Single Chiral Superfield

We propose a large class of supergravity models in terms of a single chiral matter superfield, leading to (almost) arbitrary single-field inflationary scalar potentials similar to the F-term in rigid supersymmetry. Those scalar potentials are positively semi-definite (in our approximation), and can preserve supersymmetry at the end of inflation. The only scalar superpartner of inflaton is stabilized by a higher-dimensional term in the Kahler potential. We argue that couplings of the inflaton to other sectors of the particle spectrum do not affect the inflationary dynamics, and briefly discuss reheating of the universe by the inflaton decays.

hep-th↗

arXiv2015

Reheating processes after Starobinsky inflation in old-minimal supergravity

We study reheating processes and its cosmological consequences in the Starobinsky model embedded in the old-minimal supergravity. First, we consider minimal coupling between the gravity and matter sectors in the higher curvature theory, and transform it to the equivalent standard supergravity coupled to additional matter superfields. We then discuss characteristic decay modes of the inflaton and the reheating temperature $T_{\rm R}$. Considering a simple model of supersymmetry breaking sector, we estimate gravitino abundance from inflaton decay, and obtain limits on the masses of gravitino and supersymmetry breaking field. We find $T_{\rm R}\simeq 1.0\times10^9$ GeV and the allowed range of gravitino mass as $10^4$ GeV $\lesssim m_{3/2} \lesssim 10^5$ GeV, assuming anomaly-induced decay into the gauge sector as the dominant decay channel.

hep-ph↗

arXiv2014

Inflation in Supergravity with a Single Chiral Superfield

We propose new supergravity models describing chaotic Linde- and Starobinsky-like inflation in terms of a single chiral superfield. The key ideas to obtain a positive vacuum energy during large field inflation are (i) stabilization of the real or imaginary partner of the inflaton by modifying a Kahler potential, and (ii) use of the crossing terms in the scalar potential originating from a polynomial superpotential. Our inflationary models are constructed by starting from the minimal Kahler potential with a shift symmetry, and are extended to the no-scale case. Our methods can be applied to more general inflationary models in supergravity with only one chiral superfield.

hep-th↗

arXiv2014

Complexified Starobinsky Inflation in Supergravity in the Light of Recent BICEP2 Result

Motivated by the recent observation of the B-mode signal in the cosmic microwave background by BICEP2, we stuty the Starobinsky-type inflation model in the framework of old-minimal supergravity, where the inflaton field in the original (non-supersymmetric) Starobinsky inflation model becomes a complex field. We study how the inflaton evolves on the two-dimensional field space, varying the initial condition. We show that (i) one of the scalar fields has a very steep potential once the trajectory is off from that of the original Starobinsky inflation, and that (ii) the B-mode signal observed by BICEP2 is too large to be consistent with the prediction of the model irrespective of the initial condition. Thus, the BICEP2 result strongly disfavors the complexified Starobinsky inflation in supergravity.

hep-ph↗