arXiv subjects

Kin-ya Oda

Publications and source records attributed to Kin-ya Oda.

At least 37 records · Page 2Linked to original sources

arXiv2023

Non-thermal Higgs Spectrum in Reheating Epoch: Primordial Condensate vs. Stochastic Fluctuation

Since electroweak symmetry is generally broken during inflation, the Standard Model Higgs field can become supermassive even after the end of inflation. In this paper, we study the non-thermal phase space distribution of the Higgs field during reheating, focusing in particular on two different contributions: primordial condensate and stochastic fluctuations. We obtain their analytic formulae, which agree with the previous numerical result. As a possible consequence of the non-thermal Higgs spectrum, we discuss perturbative Higgs decay during reheating for the case it is kinematically allowed. We find that the soft-relativistic and hard spectra are dominant in the decay rate of the stochastic fluctuation and that the primordial condensate and stochastic fluctuations decay almost at the same time.

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arXiv2023

Constraints on extended Jordan-Brans-Dicke gravity

Cosmological analysis of extended Jordan-Brans-Dicke (eJBD) gravity is presented in the Einstein metric frame in which gravitational interaction is readily understandable. Our formulation is the first systematic investigation of how to introduce lagrangian of standard particle physics in eJBD framework consistently with the general principle of spontaneously broken gauge symmetry, which makes it possible to confront eJBD-based cosmology with observational and laboratory bounds on time variation of parameters, masses, and coupling constants, caused by time evolution of eJBD fields. Decomposition of standard particle physics lagrangian into independent gauge invariant pieces is proposed to avoid serious conflict that may arise from standard lagrangian transformed from the Jordan frame. Independent conformal factors are assigned to each of five gauge invariant pieces. The formulation is most unambiguously made possible by defining fields having canonical kinetic terms that allow us to use the canonical quantization rule of field theory. This construction gives as one of its consequences the canonical eJBD field $\chi$ that couples to the universal fermion current, a linear combination of baryon and lepton number currents, $\partial_{\mu} \chi (\frac{1}{3} j_B^{\mu} + j_L^{\mu})$, in addition to the conventional trace of the energy-momentum tensor. Field equation of eJBD field along with gravitational equation is analyzed by using a simplified polynomial class of potential and conformal functions, giving time evolution of radiation, matter and dark energy densities consistent with observations when an appropriate set of model parameters are used. Finite temperature corrections are further calculated to give temperature dependent terms in eJBD field potential.

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arXiv2023

Gradient-flowed order parameter for spontaneous gauge symmetry breaking

The gauge-invariant two-point function of the Higgs field at the same spacetime point can make a natural gauge-invariant order parameter for spontaneous gauge symmetry breaking. However, this composite operator is ultraviolet divergent and is not well defined. We propose using a gradient flow to cure the divergence from putting the fields at the same spacetime point. As a first step, we compute it for the Abelian Higgs model with a positive mass squared at the one-loop order in the continuum theory using the saddle-point method to estimate the finite part. The order parameter consistently goes to zero in the infrared limit of the infinite flow time.

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arXiv2023

Ultraviolet Sensitivity in Higgs-Starobinsky Inflation

The general scalar-tensor theory that includes all the dimension-four terms has parameter regions that can produce successful inflation consistent with cosmological observations. This theory is in fact the same as the Higgs-Starobinsky inflation, when the scalar is identified with the Standard Model Higgs boson. We consider possible dimension-six operators constructed from non-derivative terms of the scalar field and the Ricci scalar as perturbations. We investigate how much suppression is required for these operators to avoid disrupting the successful inflationary predictions. To ensure viable cosmological predictions, the suppression scale for the sixth power of the scalar should be as high as the Planck scale. For the other terms, much smaller scales are sufficient.

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arXiv2022

Boltzmann or Bogoliubov? Approaches Compared in Gravitational Particle Production

Gravitational particle production is a minimal contribution to reheating the Universe after the end of inflation. To study this production channel, two different approaches have commonly been considered, one of which is based on the Boltzmann equation, and the other is based on the Bogoliubov transformation. Each of these has pros and cons in practice. The collision term in the Boltzmann equation can be computed based on quantum field theory in the Minkowski spacetime, and thus many techniques have been developed so far. On the other hand, the Bogoliubov approach may deal with the particle production beyond the perturbation theory and is able to take into account the effect of the curved spacetime, whereas in many cases one should rely on numerical methods, such as lattice computation. We show by explicit numerical and analytical computations of the purely gravitational production of a scalar that these two approaches give consistent results for particle production with large momenta during reheating, whereas the Boltzmann approach is not capable of computing particle production out of vacuum during inflation. We also provide analytic approximations of the spectrum of produced scalar with/without mass for the low momentum regime obtained from the Bogoliubov approach.

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arXiv2022

Double inflation via non-minimally coupled spectator

We argue that double inflation may occur when a spectator field is non-minimally coupled to gravity. As a concrete example, we study a two-field inflationary model where the initial spectator field is non-minimally coupled to gravity while the initial inflaton field is minimally coupled. The non-minimal coupling results in the growth of the spectator field which, in turn, drives the second stage of inflation in a significant region of parameter space. The isocurvature fluctuations originating from the spectator field source adiabatic ones, and hence the spectator non-minimal coupling can modify the inflationary predictions for the spectral index and the tensor-to-scalar ratio even though the initial inflaton field is minimally coupled to gravity. We explicitly show that quadratic chaotic inflation can become viable by the introduction of the spectator non-minimal coupling.

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arXiv2022

Gravitational waves in models with multicritical-point principle

The multicritical-point principle (MPP) provides a natural explanation of the large hierarchy between the Planck and electroweak scales. We consider a scenario in which MPP is applied to the Standard Model extended by two real singlet scalar fields $\phi$ and $S$, and a dimensional transmutation occurs by the vacuum expectation value of $\phi$. In this paper, we focus on the critical points that possess a $\mathbb Z_2$ symmetry $S\rightarrow -S$ and all the other fields are left invariant. Then $S$ becomes a natural dark matter (DM) candidate. Further, we concentrate on the critical points where $\phi$ does not possess further $\mathbb Z_2$ symmetry so that there is no cosmological domain-wall problem. Among such critical points, we focus on maximally critical one called CP-1234 that fix all the superrenormalizable parameters. We show that there remains a parameter region that satisfies the DM relic abundance, DM direct-detection bound and the current LHC constraints. In this region, we find a first-order phase transition in the early universe around the TeV-scale temperature. The resultant gravitational waves are predicted with a peak amplitude of ${\cal O}(10^{-12})$ at a frequency of $10^{-2}$-$10^{-1}$ Hz, which can be tested with future space-based instruments such as DECIGO and BBO.

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arXiv2022

The $R^2$-Higgs inflation with two Higgs doublets

We study $R^2$-Higgs inflation in a model with two Higgs doublets in which the Higgs sector of the Standard Model is extended by an additional Higgs doublet, thereby four scalar fields are involved in the inflationary evolutions. We first derive the set of equations required to follow the inflationary dynamics in this two Higgs doublet model, allowing a nonminimal coupling between the Higgs-squared and the Ricci scalar $R$, as well as the $R^2$ term in the covariant formalism. By numerically solving the system of equations, we find that, in parameter space where a successful $R^2$-Higgs inflation are realized and consistent with low energy constraints, the inflationary dynamics can be effectively described by a single slow-roll formalism even though four fields are involved in the model. We also argue that the parameter space favored by $R^2$-Higgs inflation requires nearly degenerate masses for $m_\mathsf{H}$, $m_A$ and $m_{\mathsf{H}^\pm}$, where $\mathsf{H}$, $A$, and $\mathsf{H}^\pm$ are the extra CP even, CP odd, and charged Higgs bosons in the general two Higgs doublet model taking renormalization group evolutions of the parameters into account. Discovery of such heavy scalars at the Large Hadron Collider (LHC) are possible if they are in the sub-TeV mass range. Indirect evidences may also emerge at the LHCb and Belle-II experiments, however, to probe the quasi degenerate mass spectra one would likely require high luminosity LHC or future lepton colliders such as the International Linear Collider and the Future Circular Collider.

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arXiv2022

Minimal scenario of Criticality for Electroweak scale, Neutrino Masses, Dark Matter, and Inflation

We propose a minimal model that can explain the electroweak scale, neutrino masses, Dark Matter (DM), and successful inflation all at once based on the multicritical-point principle (MPP). The model has two singlet scalar fields that realize an analogue of the Coleman-Weinberg mechanism, in addition to the Standard Model with heavy Majorana right-handed neutrinos. By assuming a $Z_2 $ symmetry, one of the scalars becomes a DM candidate whose property is almost the same as the minimal Higgs-portal scalar DM. In this model, the MPP can naturally realize a saddle point in the Higgs potential at high energy scales. By the renormalization-group analysis, we study the critical Higgs inflation with non-minimal coupling $ξ|H|^2 R$ that utilizes the saddle point of the Higgs potential. We find that it is possible to realize successful inflation even for $ξ=25$ and that the heaviest right-handed neutrino is predicted to have a mass around $10^{14}$ GeV to meet the current cosmological observations. Such a small value of $ξ$ can be realized by the Higgs-portal coupling $λ_{SH}\simeq 0.32$ and the vacuum expectation value of the additional neutral scalar $\langleϕ\rangle\simeq 2.7$ TeV, which correspond to the dark matter mass 2.0 TeV, its spin-independent cross section $1.8\times10^{-9}$ pb, and the mass of additional neutral scalar 190 GeV.

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arXiv2021

Echoes of 2HDM inflation at the collider experiments

We study the correlation between the constraints on general two Higgs doublet model from Higgs inflation and from collider experiments. The parameter space receives meaningful constraints from direct searches at the Large Hadron Collider and from flavor physics if $m_H$, $m_A$, and $m_{H^\pm}$ are in the sub-TeV range, where $H$, $A$, and $H^\pm$ are the CP even, CP odd, and charged Higgs bosons, respectively. We find that in the parameter region favored by the Higgs inflation, $H$, $A$, and $H^\pm$ are nearly degenerate in mass.We show that such near degeneracy can be probed directly in the upcoming runs of the Large Hadron Collider, while the future lepton colliders such as the International Linear Collider and the Future Circular Collider would provide complementary probes.

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arXiv2021

Particle decay in Gaussian wave-packet formalism revisited

We derive the Fermi's golden rule in the Gaussian wave-packet formalism of quantum field theory, proposed by Ishikawa, Shimomura, and Tobita, for the particle decay within a finite time interval. We present a systematic procedure to separate the bulk contribution from those of time boundaries, while manifestly maintaining the unitarity of the $S$-matrix unlike the proposal by Stueckelberg in 1951. We also revisit the suggested deviation from the golden rule and clarify that it indeed corresponds to the boundary contributions, though their physical significance is yet to be confirmed.

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arXiv2021

A complete set of Lorentz-invariant wave packets and modified uncertainty relation

We define a set of fully Lorentz-invariant wave packets and show that it spans the corresponding one-particle Hilbert subspace, and hence the whole Fock space as well, with a manifestly Lorentz-invariant completeness relation (resolution of identity). The position-momentum uncertainty relation for this Lorentz-invariant wave packet deviates from the ordinary Heisenberg uncertainty principle, and reduces to it in the non-relativistic limit.

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arXiv2021

New effect in wave-packet scattering of quantum fields

We report calculations of a wave-packet amplitude of the two-body scattering $\phi \phi \to \Phi \to \phi \phi$, which leads to the measured probability in realistic experiments. We elucidate the decay amplitude of $ \Phi \rightarrow \phi \phi$ from this. In such an amplitude of wave packets, there are in and out time boundaries for the initial $\Phi$ and final $\phi\phi$ configurations, respectively. In this paper, we prove that the effect of the in time boundary of $\Phi\to\phi\phi$ emerges from $\phi\phi\to\Phi\to\phi\phi$ without assuming any time boundary \emph{a priori}. This effect has been overlooked in the standard plane-wave formulation and can exhibit distinct phenomena in wide areas of science. We confirm the result in different integration orders. The result is also interpreted as a Stokes phenomenon in the Lefschetz-thimble decomposition.

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arXiv2021

Spontaneous Leptogenesis in Higgs Inflation

We propose a scenario of spontaneous leptogenesis in Higgs inflation with help from two additional operators: the Weinberg operator (Dim 5) and the derivative coupling of the Higgs field and the current of lepton number (Dim 6). The former is responsible for lepton number violation and the latter induces chemical potential for lepton number. The period of rapidly changing Higgs field, naturally realized in Higgs inflation during the reheating, allows large enhancement in the produced asymmetry in lepton number, which is eventually converted into baryon asymmetry of the universe. This scenario is compatible with high reheating temperature of Higgs inflation model.

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arXiv2021

Cosmological implications of Standard Model criticality and Higgs inflation

The observed Higgs mass indicates that the Standard Model can be valid up to near the Planck scale $M_\text{P}$. Within this framework, it is important to examine how little modification is necessary to fit the recent experimental results in particle physics and cosmology. As a minimal extension, we consider the possibility that the Higgs field plays the role of inflaton and that the dark matter is the Higgs-portal scalar field. We assume that the extended Standard Model is valid up to the string scale $10^{17}\,\text{GeV}$. (This translates to the assumption that all the non-minimal couplings are not particularly large, $ξ\lesssim 10^2$, as in the critical Higgs inflation, since $M_\text{P}/\sqrt{10^2}\sim 10^{17}\,\text{GeV}$.) We find a correlated theoretical bound on the tensor-to-scalar ratio $r$ and the dark matter mass $m_\text{DM}$. As a result, the Planck bound $r<0.09$ implies that the dark-matter mass must be smaller than 1.1\,TeV, while the PandaX-II bound on the dark-matter mass $m_\text{DM}>0.7\pm0.2\,\text{TeV}$ leads to $r\gtrsim 2\times10^{-3}$. Both are within the range of near-future detection. When we include the right-handed neutrinos of mass $M_\text{R}\sim 10^{14}$\,GeV, the allowed region becomes wider, but we still predict $r\gtrsim 10^{-3}$ in the most of the parameter space. The most conservative bound becomes $r>10^{-5}$ if we allow three-parameter tuning of $m_\text{DM}$, $M_\text{R}$, and the top-quark mass.

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arXiv2020

Dark matter in minimal dimensional transmutation with multicritical-point principle

We investigate a model with two real scalar fields that minimally generates exponentially different scales in an analog of the Coleman-Weinberg mechanism. The classical scale invariance -- the absence of dimensionful parameters in the tree-level action, required in such a scale generation -- can naturally be understood as a special case of the multipoint criticality principle. This two-scalar model can couple to the Standard Model Higgs field to realize a maximum multiplicity of criticality for field values around the electroweak scale, providing a generalization of the classical scale invariance to a wider class of criticality. As a bonus, one of the two scalars can be identified as Higgs-portal dark matter. We find that this model can be consistent with the constraints from dark matter relic abundance, its direct detection experiments, and the latest LHC data, while keeping the perturbativity up to the Planck scale. We then present successful benchmark points satisfying all these constraints: The mass of dark matter is a few TeV, and its scattering cross section with nuclei is of the order of $10^{-9}$ pb, reachable in near future experiments. The mass of extra Higgs boson $H$ is smaller than or of the order of 100 GeV, and the cross section of $e^+e^- \to ZH$ can be of fb level for collision energy 250 GeV, targetted at future lepton colliders.

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arXiv2020

Dynamically emergent gravity from hidden local Lorentz symmetry

Gravity can be regarded as a consequence of local Lorentz (LL) symmetry, which is essential in defining a spinor field in curved spacetime. The gravitational action may admit a zero-field limit of the metric and vierbein at a certain ultraviolet cutoff scale such that the action becomes a linear realization of the LL symmetry. Consequently, only three types of term are allowed in the four-dimensional gravitational action at the cutoff scale: a cosmological constant, a linear term of the LL field strength, and spinor kinetic terms, whose coefficients are in general arbitrary functions of LL and diffeomorphism invariants. In particular, all the kinetic terms are prohibited except for spinor fields, and hence the other fields are auxiliary. Their kinetic terms, including those of the LL gauge field and the vierbein, are induced by spinor loops simultaneously with the LL gauge field mass. The LL symmetry is necessarily broken spontaneously and hence is nothing but a hidden local symmetry, from which gravity is emergent.

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arXiv2020

Cosmological Perturbations in Palatini Formalism

We investigate cosmological perturbations of scalar-tensor theories in Palatini formalism. First we introduce an action where the Ricci scalar is conformally coupled to a function of a scalar field and its kinetic term and there is also a k-essence term consisting of the scalar and its kinetic term. This action has three frames that are equivalent to one another: the original Jordan frame, the Einstein frame where the metric is redefined, and the Riemann frame where the connection is redefined. For the first time in the literature, we calculate the quadratic action and the sound speed of scalar and tensor perturbations in three different frames and show explicitly that they coincide. Furthermore, we show that for such action the sound speed of gravitational waves is unity. Thus, this model serves as dark energy as well as an inflaton even though the presence of the dependence of the kinetic term of a scalar field in the non-minimal coupling, different from the case in metric formalism. We then proceed to construct the L3 action called Galileon terms in Palatini formalism and compute its perturbations. We found that there are essentially 10 different(inequivalent) definitions in Palatini formalism for a given Galileon term in metric formalism. We also see that,in general, the L3 terms have a ghost due to Ostrogradsky instability and the sound speed of gravitational waves could potentially deviate from unity, in sharp contrast with the case of metric formalism. Interestingly, once we eliminate such a ghost, the sound speed of gravitational waves also becomes unity. Thus, the ghost-free L3 terms in Palatini formalism can still serve as dark energy as well as an inflaton, like the case in metric formalism.

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