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Kiyoharu Kawana

Publications and source records attributed to Kiyoharu Kawana.

50 records · Page 3Linked to original sources
arXiv2016

Reheating-era leptogenesis

We propose a novel leptogenesis scenario at the reheating era. Our setup is minimal in the sense that, in addition to the standard model Lagrangian, we only consider an inflaton and higher dimensional operators. The lepton number asymmetry is produced not by the decay of a heavy particle, but by the scattering between the standard model particles. After the decay of an inflaton, the model is described within the standard model with higher dimensional operators. The Sakharov's three conditions are satisfied by the following way. The violation of the lepton number is realized by the dimension-5 operator. The complex phase comes from the dimension-6 four lepton operator. The universe is out of equilibrium before the reheating is completed. It is found that the successful baryogenesis is realized for the wide range of parameters, the inflaton mass and reheating temperature, depending on the cutoff scale. Since we only rely on the effective Lagrangian, our scenario can be applicable to all mechanisms to generate neutrino Majorana masses.

hep-ph↗
arXiv2016

Possible explanations for fine-tuning of the universe

The Froggatt-Nielsen mechanism and the multi-local field theory are interesting and promising candidates for solving the naturalness problem in the universe. These theories are based on the different physical principles: The former assumes the micro-canonical partition function $\int {\cal{D}}ϕ \prod_i δ(S_i^{}-I_i^{})$, and the latter assumes the partition function $\int {\cal{D}}ϕ \exp\left(iS_M^{}\right)$ where $S_M^{}$ is the multi-local action $\sum_i c_i^{}S_i^{}+\sum_{i,j}c_{i,j}^{}S_i^{}S_j^{}+\cdots $. Our main purpose is to show that they are equivalent in the sense that they predict the same fine-tuning mechanism. In order to clarify our argument, we first study (review) the similarity between the Froggatt-Nielsen mechanism and statistical mechanics in detail, and show that the dynamical fine-tuning in the former picture can be understood completely in the same way as the determination of the temperature in the latter picture. Afterward, we discuss the multi-local theory and the equivalence between it and the the Froggatt-Nielsen mechanism. Because the multi-local field theory can be obtained from physics at the Planck/String scale, this equivalence indicates that the micro-canonical picture can also originate in such physics. As a concrete example, we also review the IIB matrix model as an origin of the multi-local theory.

hep-th↗
arXiv2016

Models of LHC Diphoton Excesses Valid up to the Planck scale

We discuss a possibility to explain the LHC diphoton excesses at $750$GeV by the new scalar $X$ that couples to the gauge bosons through the loop of new massive particles with Standard Model charges. We assume that the new particles decay into the Standard Model particles at the tree level. We systematically examine the models that preserve the vacuum stability and the perturbativity up to the Planck scale. When we take scalars for the new particles, we find that only a few diquark and dilepton models can explain the observed diphoton cross section without conflicting the experimental mass bounds. When we take vector-like fermions for the new particles, we find rather different situations depending on whether their couplings to $X$ are scalar or pseudoscalar type. In the former case, a few models are allowed if we introduce only one species of fermions. The more fermions we introduce, the more models are allowed. In the later case, the most of the models are allowed because of the large coupling between $X$ and photon. It is interesting that the allowed mass regions of the scalar particles might be reached by the next lepton colliders.

hep-ph↗
arXiv2015

Vanishing Higgs Potential in Minimal Dark Matter Models

We consider the Standard Model with a new particle which is charged under $SU(2)_{L}$ with the hypercharge being zero. Such a particle is known as one of the dark matter (DM) candidates. We examine the realization of the multiple point criticality principle (MPP) in this class of models. Namely, we investigate whether the one-loop effective Higgs potential $V_{\text{eff}}(ϕ)$ and its derivative $dV_{\text{eff}}(ϕ)/dϕ$ can become simultaneously zero at around the string/Planck scale, based on the one/two-loop renormalization group equations. As a result, we find that only the $SU(2)_L$ triplet extensions can realize the MPP. More concretely, in the case of the triplet Majorana fermion, the MPP is realized at the scale $ϕ={\cal{O}}(10^{16}\text{GeV})$ if the top mass $M_{t}$ is around $172$ GeV. On the other hand, for the real triplet scalar, the MPP can be satisfied for $10^{16}\text{ GeV}\lesssimϕ\lesssim10^{17}$GeV and $172\text{ GeV}\gtrsim M_{t}\gtrsim171$ GeV, depending on the coupling between the Higgs and DM.

hep-ph↗
arXiv2015

Natural solution to the naturalness problem -- Universe does fine-tuning

We propose a new mechanism to solve the fine-tuning problem. We start from a multi-local action $ S=\sum_{i}c_{i}S_{i}+\sum_{i,j}c_{i,j}S_{i}S_{j}+\sum_{i,j,k}c_{i,j,k}S_{i}S_{j}S_{k}+\cdots$, where $S_{i}$'s are ordinary local actions. Then, the partition function of this system is given by \begin{equation} Z=\int d\overrightarrowλ f(\overrightarrowλ)\langle f|T\exp\left(-i\int_{0}^{+\infty}dt\hat{H}(\overrightarrowλ;a_{cl}(t))\right)|i\rangle,\nonumber\end{equation} where $\overrightarrowλ$ represents the parameters of the system whose Hamiltonian is given by $\hat{H}(\overrightarrowλ;a_{cl}(t))$, $a_{cl}(t)$ is the radius of the universe determined by the Friedman equation, and $f(\overrightarrowλ)$, which is determined by $S$, is a smooth function of $\overrightarrowλ$. If a value of $\overrightarrowλ$, $\overrightarrowλ_{0}$, dominates in the integral, we can interpret that the parameters are dynamically tuned to $\overrightarrowλ_{0}$. We show that indeed it happens in some realistic systems. In particular, we consider the strong CP problem, multiple point criticality principle and cosmological constant problem. It is interesting that these different phenomena can be explained by one mechanism.

hep-th↗
arXiv2015

Saddle point inflation in string-inspired theory

The observed value of the Higgs mass indicates the possibility that there is no supersymmetry below the Planck scale and that the Higgs can play the role of the inflaton. We examine the general structure of the saddle point inflation in string-inspired theory without supersymmetry. We point out that the string scale is fixed to be around the GUT scale $\sim10^{16}$GeV in order to realize successful inflation. We find that the inflaton can be naturally identified with the Higgs field.

hep-ph↗
arXiv2015

Criticality and Inflation of the Gauged B-L Model

We consider the multiple point principle (MPP) and the inflation of the gauged B-L extension of the Standard Model (SM) with a classical conformality. We examine whether the scalar couplings and their beta functions can become simultaneously zero at $Λ_{\text{MPP}}:=10^{17}$ GeV by using the two-loop renormalization group equations (RGEs). We find that we can actually realize such a situation and that the parameters of the model are uniquely determined by the MPP. However, as discussed in \cite{Iso:2012jn}, if we want to realize the electroweak symmetry breaking by the radiative B-L symmetry breaking, the self coupling $λ_Ψ$ of a newly introduced SM singlet complex scalar $Ψ$ must have a non-zero value at $Λ_{\text{MPP}}$, which means the breaking of the MPP. We find that the ${\cal{O}}(100)$GeV electroweak symmetry breaking can be achieved even if this breaking is very small; $λ_Ψ(Λ_{\text{MPP}})\leq10^{-10}$. Within this situation, the mass of the B-L gauge boson is predicted to be \begin{equation} M_{B-L}=2\sqrt{2}\times\sqrt{\frac{λ(v_{h})}{0.10}}\times v_{h}\simeq 696\hspace{1mm}\text{GeV},\nonumber\end{equation} where $λ$ is the Higgs self coupling and $v_{h}$ is the Higgs expectation value. This is a remarkable prediction of the (slightly broken) MPP. Furthermore, such a small $λ_Ψ$ opens a new possibility: $Ψ$ plays a roll of the inflaton \cite{Okada:2011en}. Another purpose of this paper is to investigate the $λ_ΨΨ^{4}$ inflation scenario with the non-minimal gravitational coupling $ξΨ^{2} {\cal{R}}$ based on the two-loop RGEs.

hep-ph↗
arXiv2015

Reconsideration of the Coleman's Baby Universe

We reconsider the Coleman's mechanism that solves the Cosmological Constant Problem by the baby universes. We use the Lorentzian path integral and allow each universe has a different effective field theory and a vacuum. By using the probability distribution of coupling constants, it is shown that the cosmological constant of our universe does not necessary become small due to the effects of other universes.

hep-th↗
arXiv2015

Landau pole in the Standard Model with weakly interacting scalar fields

We consider the Standard Model with a new scalar field $X$ which is a $n_X^{}$ representation of the $SU(2)_L$ with a hypercharge $Y_X$. The renormalization group running effects on the new scalar quartic coupling constants are evaluated. Even if we set the scalar quartic coupling constants to be zero at the scale of the new scalar field, the coupling constants are induced by the one-loop effect of the weak gauge bosons. Once non-vanishing couplings are generated, the couplings rapidly increase by renormalization group effect of the quartic coupling constant itself. As a result, the Landau pole appears below Planck scale if $n_X^{}\geq 4$. We find that the scale of the obtained Landau pole is much lower than that evaluated by solving the one-loop beta function of the gauge coupling constants.

hep-ph↗
arXiv2015

Multiple Point Principle of the Gauged B-L Model

We consider the multiple point principe (MPP), its breaking and the inflation of the gauged B-L model. Our results are based on the two-loop renormalization group equations (RGEs). We find that this model can realize the flat potentials at $Λ_{\text{MPP}}=10^{17}$GeV, and that the unknown parameters of the model are fixed by the MPP. Furthermore, we show that it is possible to realize both of the electroweak symmetry breaking at ${\cal{O}}(100)$GeV and the inflation by breaking the MPP.

hep-ph↗
arXiv2015

Weak Scale From the Maximum Entropy Principle

The theory of multiverse and wormholes suggests that the parameters of the Standard Model are fixed in such a way that the radiation of the $S^{3}$ universe at the final stage $S_{rad}$ becomes maximum, which we call the maximum entropy principle. Although it is difficult to confirm this principle generally, for a few parameters of the Standard Model, we can check whether $S_{rad}$ actually becomes maximum at the observed values. In this paper, we regard $S_{rad}$ at the final stage as a function of the weak scale ( the Higgs expectation value ) $v_{h}$, and show that it becomes maximum around $v_{h}={\cal{O}}(300\text{GeV})$ when the dimensionless couplings in the Standard Model, that is, the Higgs self coupling, the gauge couplings, and the Yukawa couplings are fixed. Roughly speaking, we find that the weak scale is given by \begin{equation} v_{h}\sim\frac{T_{BBN}^{2}}{M_{pl}y_{e}^{5}},\nonumber\end{equation} where $y_{e}$ is the Yukawa coupling of electron, $T_{BBN}$ is the temperature where the Big Bang Nucleosynthesis starts and $M_{pl}$ is the Planck mass.

hep-ph↗
arXiv2015

Multiple Point Principle of the Standard Model with Scalar Singlet Dark Matter and Right Handed Neutrinos

We consider the multiple point principle (MPP) of the Standard Model (SM) with the scalar singlet Dark Matter (DM) and three heavy right-handed neutrinos at the scale where the beta function $β_λ$ of the effective Higgs self coupling $λ_{\text{eff}}$ becomes zero. We make the two-loop analysis and find that the top quark mass $M_{t}$ and the Higgs portal coupling $κ$ are strongly related each other. One of the good points in this model is that the larger $M_{t}{1mm}(\gtrsim 171\text{GeV})$ is allowed. This fact is consistent with the recent experimental value \cite{ATLAS:2014wva} $M_{t}=173.34\pm0.76$ GeV, which corresponds to the DM mass $769{1mm}\text{GeV}\leq m_{\text{DM}}\leq 1053 {1mm}\text{GeV}$.

hep-ph↗
arXiv2014

Evidence of the Big Fix

We give an evidence of the Big Fix. The theory of wormholes and multiverse suggests that the parameters of the Standard Model are fixed in such a way that the total entropy at the late stage of the universe is maximized, which we call the maximum entropy principle. In this paper, we discuss how it can be confirmed by the experimental data, and we show that it is indeed true for the Higgs vacuum expectation value $v_{h}$. We assume that the baryon number is produced by the sphaleron process, and that the current quark masses, the gauge couplings and the Higgs self coupling are fixed when we vary $v_{h}$. It turns out that the existence of the atomic nuclei plays a crucial role to maximize the entropy. This is reminiscent of the anthropic principle, however it is required by the fundamental low in our case.

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