arXiv subjects

Jisuke Kubo

Publications and source records attributed to Jisuke Kubo.

At least 19 recordsLinked to original sources

arXiv2026

Dark matter in scale-invariant gravity with hidden-sector condensation

The origin of the electroweak scale, cosmic inflation, and dark matter are often treated as independent problems beyond the Standard Models of particle physics and cosmology. In this work, we explore the possibility that they instead arise from a common underlying framework based on classically scale-invariant quadratic gravity coupled to a strongly interacting hidden sector. The $R^2$ term naturally realizes Starobinsky inflation, while confinement in the hidden sector dynamically generates the Planck scale and triggers electroweak symmetry breaking through a gravitationally induced Higgs mass generation mechanism. The scalar degree of freedom associated with the $R^2$ term subsequently reheats both the visible and hidden sectors through universal couplings to the energy-momentum tensor, leading to the gravitational freeze-in production of hidden-sector states. We investigate three representative realizations of the hidden sector in which the dark matter candidate is either a hidden $\eta'$ meson, a hidden vector boson, or charged hidden pions, and derive the corresponding dark matter and dark radiation relic abundances.

hep-ph↗

arXiv2025

Gravity and the Hierarchy Problem

We propose a mechanism where the dynamical generation of the Planck mass in scale invariant gravity leads to Einstein gravity, successful inflation and an explanation of the hierarchy problem of the Standard Model. We will discuss the scale generation by dynamical symmetry breaking and phenomenological consequences.

hep-ph↗

arXiv2025

Primordial Gravitational Waves in Quadratic Gravity

Quadratic gravity is a fourth-order (in derivatives) theory that can serve as an attractive upgrade to the standard description of gravity provided by General Relativity, thanks to its renormalizability and its built-in description of primordial inflation. We bring quadratic gravity into a second-order form by introducing an auxiliary tensor field and we consider the primordial tensor fluctuations (gravitational waves) in the theory around a Friedmann-Lema\^itre-Robertson-Walker background. After a canonical quantization of the perturbations, we calculate the tensor power spectrum in quasi de Sitter spacetime. We find that the spectral index $n_t$ and the amplitude $A_t$ of the tensor power spectrum are both suppressed by the factor $(1 + 2{\bf H}^2_*/m_\text{gh}^2)^{-1}$, where ${\bf H}_*$ is the Hubble rate at horizon exit and $m_\text{gh}$ is the mass of the spin-two ghost. This restores the slow-roll consistency condition familiar from single-field inflation models, where the tensor-to-scalar ratio $r$ is equal to $-8n_t$ in the lowest nontrivial order in the slow-roll approximation. We also discuss the well-known issue of the ghost problem in fourth-order theories and how it pertains to the results at hand.

gr-qc↗

arXiv2024

Anti-Instability of Complex Ghost

We argue that Lee-Wick's complex ghost appearing in any higher derivative theory is stable and its asymptotic field exists. It may be more appropriate to call it ``anti-unstable" in the sense that, the more the ghost `decays' into lighter ordinary particles, the larger the probability the ghost remains as itself becomes. This is explicitly shown by analyzing the two-point functions of the ghost Heisenberg field which is obtained as an exact result in the $N\rightarrow\infty$ limit in a massive scalar ghost theory with light $O(N)$-vector scalar matter. The anti-instability is a consequence of the fact that the poles of the complex ghost propagator are located on the physical sheet in the complex plane of four-momentum squared. This should be contrasted to the case of the ordinary unstable particle, whose propagator has no pole on the physical sheet.

hep-th↗

arXiv2024

Scale Invariant Extension of the Standard Model: A Nightmare Scenario in Cosmology

Inflationary observables of a classically scale invariant model, in which the origin of the Planck mass and the electroweak scale including the right-handed neutrino mass is chiral symmetry breaking in a QCD-like hidden sector, are studied. Despite a three-field inflation the initial-value-dependence is strongly suppressed thanks to a river-valley like potential. The model predicts the tensor-to-scalar ratio $r$ of cosmological perturbations smaller than that of the $R^2$ inflation, i.e., $ 0.0044 \gsim r \gsim 0.0017$ for e-foldings between $50$ and $60$: The model will be consistent even with a null detection at LiteBird/CMB-S4. We find that the non-Gaussianity parameter $f_{NL}$ is $O(10^{-2})$, the same size as that of single-field inflation. The dark matter particles are the lightest Nambu-Goldstone bosons associated with chiral symmetry breaking, which are decay products of one of the inflatons and are heavier than $10^9$ GeV with a strongly suppressed coupling with the standard model, implying that the dark matter will be unobservable in direct as well as indirect measurements.

hep-ph↗

arXiv2023

Unitarity Violation in Field Theories of Lee-Wick's Complex Ghost

Theories with fourth-order derivatives, including the Lee-Wick finite QED model and Quadratic Gravity, have a better UV behaviour, but the presence of negative metric ghost modes endanger unitarity. Noticing that the ghost acquires a complex mass by radiative corrections, Lee and Wick, in particular, claimed that such complex ghosts would never be created by collisions of physical particles because of energy conservation, so that the physical S-matrix unitarity must hold. We investigate the unitarity problem faithfully working in the operator formalism of quantum field theory. When complex ghosts participate, a complex delta function (generalization of Dirac delta function) appears at each interaction vertex, which enforces a specific conservation law of complex energy. Its particular property implies that the naive Feynman rule is wrong if the four-momenta are assigned to the internal lines after taking account of the conservation law in advance. We show that the complex ghosts are actually created and unitarity is violated in such fourth-order derivative theories. We also find a definite energy threshold below which the ghosts cannot be created: The theories are unitary and renormalizable below the threshold.

hep-th↗

arXiv2022

Inflation with Massive Spin-2 Ghosts

We consider a generic model of quadratic gravity coupled to a single scalar and investigate the effects of gravitational degrees of freedom on inflationary parameters. We find that quantum corrections arising from the massive spin-2 ghost generate significant contributions to the effective inflationary potential and allow for a realization of the spontaneous breakdown of global scale invariance without the need for additional scalar fields. We compute inflationary parameters, compare the resulting predictions to well-known inflationary models, and find that they fit well within the Planck and BICEP/Keck collaboration's constraints on inflation.

astro-ph.CO↗

arXiv2022

Spontaneous Conformal Symmetry Breaking and Quantum Quadratic Gravity

We investigate several quantum phenomena related to quadratic gravity after rewriting the general fourth-order action in a more convenient form that is second-order in derivatives and produces only first-class constraints in phase space. We find that a Higgs mechanism may occur in the conformally invariant subset of the general quadratic action if the theory is conformally coupled to a scalar field that acquires a non-zero vacuum expectation value and spontaneously breaks the conformal symmetry. Then, in the broken phase, the originally massless spin-2 ghost may absorb both the scalar and vector fields to become massive. We also perform a BRST quantization of second-order quadratic gravity in the covariant operator formalism and discuss conditions under which unitarity of the full interacting quantum theory may be established.

hep-th↗

arXiv2022

Analysis of Unitarity in Conformal Quantum Gravity

We perform a canonical quantization of Weyl's conformal gravity by means of the covariant operator formalism and investigate the unitarity of the resulting quantum theory. After reducing the originally fourth order theory to second order in time derivatives via the introduction of an auxiliary tensor field, we identify the full Fock space of quantum states under a BRST construction that includes Faddeev-Popov ghost fields corresponding to Weyl transformations. Using the Kugo-Ojima quartet mechanism, we identify the physical subspace of quantum states and find that the subspace containing the transverse spin-2 states comes equipped with an indefinite inner product metric and a one-particle Hamiltonian that possesses only a single eigenstate. We construct the LSZ reduction formula for the S-matrix in this spin-2 subspace and find that unitarity is violated in scattering events. The explicit way in which this violation occurs represents a new view on the ghost-problem in quadratic theories of quantum gravity.

hep-th↗

arXiv2022

Inflation and dark matter after spontaneous Planck scale generation by hidden chiral symmetry breaking

Dynamical chiral symmetry breaking in a QCD-like hidden sector is used to generate the Planck mass and the electroweak scale including the heavy right-handed neutrino mass. A real scalar field transmits the energy scale of the hidden sector to the visible sectors, playing besides a role of inflaton in the early Universe while realizing a Higgs-inflation-like model. Our dark matter candidates are hidden pions that raise due to dynamical chiral symmetry breaking. They are produced from the decay of inflaton. Unfortunately, it will be impossible to directly detect them, because they are super heavy ($10^{9\,\sim\,12}$ GeV), and moreover the interaction with the visible sector is extremely suppressed.

hep-ph↗

arXiv2021

Unified Emergence of Energy Scales and Cosmic Inflation

In the quest for unification of the Standard Model with gravity, classical scale invariance can be utilized to dynamically generate the Planck mass $M_\mathrm{Pl}$. Then, the relation of Planck scale physics to the scale of electroweak symmetry breaking $μ_H$ requires further explanation. In this paper, we propose a model that uses the spontaneous breaking of scale invariance in the scalar sector as a unified origin for dynamical generation of both scales. Using the Gildener-Weinberg approximation, only one scalar acquires a vacuum expectation value of $v_S \sim (10^{16-17})\,\mathrm{GeV}$, thus radiatively generating $M_\mathrm{Pl} \approx β_S^{1/2} v_S$ and $μ_H$ via the neutrino option with right handed neutrino masses $m_N = y_M v_S \sim 10^7 \,\mathrm{GeV}$. Consequently, active SM neutrinos are given a mass with the inclusion of a type-I seesaw mechanism. Furthermore, we adopt an unbroken $Z_2$ symmetry and a $Z_2$-odd set of right-handed Majorana neutrinos $χ$ that do not take part in the neutrino option and are able to produce the correct dark matter relic abundance (dominantly) via inflaton decay. The model also describes cosmic inflation and the inflationary CMB observables are predicted to interpolate between those of $R^2$ and linear chaotic inflationary model and are thus well within the strongest experimental constraints.

hep-ph↗

arXiv2021

Semi-secretly interacting ALP as an explanation of Fermilab muon $g-2$ measurement

The muon anomalous magnetic moment measurement has, for more than a decade, been a long-standing anomaly hinting the physics beyond the Standard Model (BSM). The recently announced results from muon $g-2$ collaboration, corresponding to 3.3$σ$ deviation from Standard Model value (4.2$σ$ in combination with previous measurement) are strengthening the need for new physics coupled to muons. In this letter, we propose a novel scenario in which Standard Model (SM) is augmented by an axion-like particle (ALP) and vector-like fermions. We find that such a model admits an excellent interpretation of recent muon $g-2$ measurement through quantum process featuring ALP interacting with muons and newly introduced fermions. Previously proposed explanations with ALPs utilize interactions with photons and/or SM fermions. Therefore, in this letter we complement and extend such scenarios. We also discuss collider prospects for the model as well as the possibility that ALP is long lived or stable dark matter (DM) candidate.

hep-ph↗

arXiv2020

Heavy Dark Matter, Neutrino Masses and Higgs Naturalness from a Strongly Interacting Hidden Sector

We consider the extension of the Standard Model (SM) with a strongly interacting QCD-like hidden sector, at least two generations of right-handed neutrinos and one scalar singlet. Once scalar singlet obtains a nonzero vacuum expectation value, active neutrino masses are generated through type-I seesaw mechanism. Simultaneously, the electroweak scale is generated through the radiative corrections involving these massive fermions. This is the essence of the scenario that is known as the "neutrino option" for which the successful masses of right-handed neutrinos are in the range $10^7-10^8$ GeV. The main goal of this work is to scrutinize the potential to accommodate dark matter in such a realization. The dark matter candidates are Nambu-Goldstone bosons which appear due to the dynamical breaking of the hidden chiral symmetry. The mass spectrum studied in this work is such that masses of Nambu-Goldstone bosons and singlet scalar exceed those of right-handed neutrinos. Having the masses of all relevant particles several orders of magnitude above $\mathcal{O}$(TeV), the freeze-out of dark matter is not achievable and hence we turn to alternative scenarios, namely freeze-in. The Nambu-Goldstone bosons can interact with particles that are not in SM but, however, have non-negligible abundance through their not-too-small couplings with SM. Utilizing this, we demonstrate that the dark matter in the model is successfully produced at temperature scale where the right-handed neutrinos are still stable. We note that the lepton number asymmetry sufficient for the generation of observable baryon asymmetry of the Universe can be produced in right-handed neutrino decays. Hence, we infer that the model has the potential to simultaneously address several of the most relevant puzzles in contemporary high-energy physics.

hep-ph↗

arXiv2020

Gravitational waves from chiral phase transition in a conformally extended standard model

The gravitational wave (GW) background produced at the cosmological chiral phase transition in a conformal extension of the standard model is studied. To obtain the bounce solution of coupled field equations we implement an iterative method. We find that the corresponding $O(3)$ symmetric Euclidean action $S_3$ divided by the temperature $T$ has a simple behavior near the critical temperature $T_C$: $S_3/T \propto (1-T/T_C)^{-γ}$, which is subsequently used to determine the transition's inverse duration $β$ normalized to the Hubble parameter $H$. It turns out that $β/H \gtrsim 10^3$, implying that the sound wave period $τ_\text{sw}$ as an active GW source, too, can be much shorter than the Hubble time. We therefore compute $τ_\text{sw} H$ and use it as the reduction factor for the sound wave contribution. The signal-to-noise ratio (SNR) for Deci-Hertz Interferometer Gravitational Wave Observatory (DECIGO) and Big Bang Observer (BBO) is evaluated, with the result: SNR$^\text{DECIGO} \lesssim 1.2$ and SNR$^\text{BBO} \lesssim 12.0$ for five years observation, from which we conclude that the GW signal predicted by the model in the optimistic case could be detected at BBO.

hep-ph↗

arXiv2019

Observational prospects for gravitational waves from hidden or dark chiral phase transitions

We study the gravitational wave (GW) signature of first-order chiral phase transitions ($χ$PT) in strongly interacting hidden or dark sectors. We do so using several effective models in order to reliably capture the relevant non-perturbative dynamics. This approach allows us to explicitly calculate key quantities characterizing the $χ$PT without having to resort to rough estimates. Most importantly, we find that the transition's inverse duration $β$ normalized to the Hubble parameter $H$ is at least two orders of magnitude larger than typically assumed in comparable scenarios, namely $β/H\gtrsim\mathcal{O}(10^4)$. The obtained GW spectra then suggest that signals from hidden $χ$PTs occurring at around 100 MeV can be in reach of LISA, while DECIGO and BBO may detect a stochastic GW background associated with transitions between roughly 1 GeV and 10 TeV. Signatures of transitions at higher temperatures are found to be outside the range of any currently proposed experiment. Even though predictions from different effective models are qualitatively similar, we find that they may vary considerably from a quantitative point of view, which highlights the need for true first-principle calculations such as lattice simulations.

hep-ph↗

arXiv2019

Planck mass and inflation as consequences of dynamically broken scale invariance

Classical scale invariance represents a promising framework for model building beyond the Standard Model. However, once coupled to gravity, any scale-invariant microscopic model requires an explanation for the origin of the Planck mass. In this paper, we provide a minimal example for such a mechanism and show how the Planck mass can be dynamically generated in a strongly coupled gauge sector. We consider the case of hidden SU(N_c) gauge interactions that link the Planck mass to the condensation of a scalar bilinear operator that is nonminimally coupled to curvature. The effective theory at energies below the Planck mass contains two scalar fields: the pseudo-Nambu--Goldstone boson of spontaneously broken scale invariance (the dilaton) and a gravitational scalar degree of freedom that originates from the R^2 term in the effective action (the scalaron). We compute the effective potential for the coupled dilaton-scalaron system at one-loop order and demonstrate that it can be used to successfully realize a stage of slow-roll inflation in the early Universe. Remarkably enough, our predictions for the primordial scalar and tensor power spectra interpolate between those of standard R^2 inflation and linear chaotic inflation. For comparatively small gravitational couplings, we thus obtain a spectral index n_s ~= 0.97 and a tensor-to-scalar ratio as large as r ~= 0.08.

hep-ph↗

arXiv2019

Gravitational Waves from First-Order Phase Transitions: LIGO as a Window to Unexplored Seesaw Scales

Within a recently proposed classically conformal model, in which the generation of neutrino masses is linked to spontaneous scale symmetry breaking, we investigate the associated phase transition and find it to be of strong first order with a substantial amount of supercooling. Carefully taking into account the vacuum energy of the metastable minimum, we demonstrate that a significant fraction of the model's parameter space can be excluded simply because the phase transition cannot complete. We argue this to be a powerful consistency check applicable to general theories based on classical scale invariance. Finally, we show that all remaining parameter points predict a sizable gravitational wave signal, so that the model can be fully tested by future gravitational wave observatories. In particular, most of the parameter space can already be probed by the upcoming LIGO science run starting in early 2019.

hep-ph↗

arXiv2018

Scale and confinement phase transitions in scale invariant $SU(N)$ scalar gauge theory

We consider scalegenesis, spontaneous scale symmetry breaking, by the scalar-bilinear condensation in $SU(N)$ scalar gauge theory. In an effective field theory approach to the scalar-bilinear condensation at finite temperature, we include the Polyakov loop to take into account the confinement effect. The theory with $N=3,4,5$ and $6$ is investigated, and we find that in all these cases the scale phase transition is a first-order phase transition. We also calculate the latent heat at and slightly below the critical temperature. Comparing the results with those obtained without the Polyakov loop effect, we find that the Polyakov effect can considerably increase the latent heat in some cases, which would mean a large increase in the energy density of the gravitational waves background, if it were produced by the scale phase transition.

hep-th↗