arXiv subjects

Kunio Kaneta

Publications and source records attributed to Kunio Kaneta.

At least 37 records · Page 2Linked to original sources

arXiv2023

Gravitational and dark wave emission at binary merger

The recently proposed formalism of extended Jordan-Brans-Dicke gravity makes it possible to calculate energy loss rate due to both gravitational wave and scalar field (giving the origin of dark energy) wave emission at merger of a black hole and a neutron star; a binary system of no scalar hair and a star with the scalar charge. The scalar field emission changes orbit parameters of the binary system, thereby changes detectable gravitational wave emission. When neutron stars carry significantly large scalar charge, significant dark wave (namely, scalar field wave) emission occurs at the same time of gravitational wave emission. It is found that solutions of coupled differential equations predict non-vanishing remnant dark charge after the gravitational collapse. This gives two interesting possibilities: (1) the no-hair conjecture of black hole is violated, or (2) a bosonic cloud is formed outside the event horizon of black hole. The bosonic cloud proposed in the literature is a gigantic atom made of gravitationally bound dark energy quanta surrounding a black hole. One can either constrain, or even determine, parameters of extended Jordan-Brans-Dicke gravity from accumulated gravitational wave observations of merging black hole and neutron star.

gr-qc↗

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.

hep-ph↗

arXiv2023

Quantifying Limits on CP Violating Phases from EDMs in Supersymmetry

We revisit the calculation of the electron, neutron, and proton electric dipole moments (EDMs) in the constrained minimal supersymmetric standard model (CMSSM). The relatively large mass of the Higgs boson, $m_H \simeq 125$ GeV coupled with the (as yet) lack of discovery of any supersymmetric particle at the LHC, has pushed the supersymmetry breaking scale to several TeV or higher. Though one might expect this decoupling to have relaxed completely any bounds on the two CP violating phases in the CMSSM ($θ_μ$ and $θ_A$), the impressive experimental improvements in the limits on the EDMs (particularly the electron EDM) still allow us to set constraints of order $(0.01 - 0.1)π$ on $θ_A$ and $(0.001 - 0.1)π$ on $θ_μ$. We also discuss the impact of future improvements in the experimental limits on supersymmetric models.

hep-ph↗

arXiv2023

Anomalous and axial Z' contributions to g-2

We study the effects of an anomalous Z' boson on the anomalous magnetic moment of the muon (g-2), and especially the impact of its axial coupling. We mainly evaluate the negative contribution to (g-2) of such couplings at one-loop and look at the anomalous couplings generated at two loops. We find areas of the parameter space, where the anomalous contribution becomes comparable and even dominant compared to the one-loop contribution. We show that in such cases, the cutoff of the theory is sufficiently low, so that new charged fermions can be found in the next round of collider experiments. We comment on the realization of such a context in string theory orientifolds.

hep-ph↗

arXiv2023

Gauged Quintessence

Despite its dominance in the present universe's energy budget, dark energy is the least understood component in the universe. Although there is a popular model for the dynamical dark energy, the quintessence scalar, the investigation is limited because of its highly elusive character. We present a model where the quintessence is gauged by an Abelian gauge symmetry. The quintessence is promoted to be a complex scalar whose real part is the dark energy field while the imaginary part is the longitudinal component of a new gauge boson. It brings interesting characters to dark energy physics. We study the general features of the model, including how the quintessence behavior is affected and how the solicited dark energy properties constrain its gauge interaction. We also note that while the uncoupled quintessence models are suffered greatly from the Hubble tension, it can be alleviated if the quintessence is under the gauge symmetry.

astro-ph.CO↗

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.

astro-ph.CO↗

arXiv2021

Conformal Portal to Dark Matter

We propose a new portal coupling to dark matter by taking advantage of the nonminimally coupled portal sector to the Ricci scalar. Such a portal sector conformally induces couplings to the trace of the energy-momentum tensor of matters including highly secluded dark matter particles. The portal coupling is so feeble that dark matter is produced by freeze-in processes of scatterings and/or the decay of the mediator. We consider two concrete realizations of the portal: conformally induced Higgs portal and conformally induced mediator portal. The former case is compatible with the Higgs inflation, while the latter case can be tested by dark matter direct detection experiments.

hep-ph↗

arXiv2021

Freeze-in from Preheating

We consider the production of dark matter during the process of reheating after inflation. The relic density of dark matter from freeze-in depends on both the energy density and energy distribution of the inflaton scattering or decay products composing the radiation bath. We compare the perturbative and non-perturbative calculations of the energy density in radiation. We also consider the (likely) possibility that the final state scalar products are unstable. Assuming either thermal or non-thermal energy distribution functions, we compare the resulting relic density based on these different approaches. We show that the present-day cold dark matter density can be obtained through freeze-in from preheating for a large range of dark matter masses.

hep-ph↗

arXiv2021

Metastable Conformal Dark Matter

We show that a metastable dark matter candidate arises naturally from the conformal transformation between the Einstein metric, where gravitons are normalised states, and the Jordan metric dictating the coupling between gravity and matter. Despite being secluded from the Standard Model by a large scale above which the Jordan metric shows modifications to the Einstein frame metric, dark matter couples to the energy momentum tensor of the Higgs field in the primordial plasma primarily. This allows for the production of dark matter in a sufficient amount which complies with observations. The seclusion of dark matter makes it long-lived for masses $\lesssim 1$ MeV, with a lifetime much above the age of the Universe and the present experimental limits. Such a dark matter scenario has clear monochromatic signatures generated by the decay of the dark matter candidate into neutrino and/or $γ-$rays.

hep-ph↗

arXiv2021

Inflaton Oscillations and Post-Inflationary Reheating

We analyze in detail the perturbative decay of the inflaton oscillating about a generic form of its potential $V(ϕ) = ϕ^k$, taking into account the effects of non-instantaneous reheating. We show that evolution of the temperature as a function of the cosmological scale factor depends on the spin statistics of the final state decay products when $k > 2$. We also include the inflaton-induced mass of the final states leading to either kinematic suppression or enhancement if the final states are fermionic or bosonic respectively. We compute the maximum temperature reached after inflation, the subsequent evolution of the temperature and the final reheat temperature. We apply our results to the computation of the dark matter abundance through thermal scattering during reheating. We also provide an example based on supersymmetry for the coupling of the inflaton to matter.

hep-ph↗

arXiv2021

Energy-Momentum portal to dark matter and emergent gravity

We propose a new scenario where dark matter belongs to a secluded sector coupled to the Standard Model through energy--momentum tensors. Our model is motivated by constructions where gravity {\it emerges} from a hidden sector, the graviton being identified by the kinetic term of the fields in the secluded sector. Supposing that the lighter particle of the secluded sector is the dark component of the Universe, we show that we can produce it in a sufficiently large amount despite the suppressed couplings of the theory, thanks to large temperatures of the thermal bath in the early stage of the Universe.

hep-ph↗

arXiv2020

Disformal Dark Matter

We generalize dark matter production to a two-metric framework whereby the physical metric, which couples to the Standard Model (SM), is conformally and/or disformally related to the metric governing the gravitational dynamics. We show that this setup is naturally present in many Ultra Violet (UV) constructions, from Kähler moduli fields to tensor-portal models, and from emergent gravity to supergravity models. In this setting we study dark matter production in the early Universe resulting from both scatterings off the thermal bath and the radiative decay of the inflaton. We also take into account non-instantaneous reheating effects at the end of inflation. In this context, dark matter emerges from the production of the scalar field mediating the conformal/disformal interactions with the SM, i.e. realising a Feebly Interacting Matter Particle (FIMP) scenario where the suppression scale of the interaction between the scalar and the SM can be taken almost as high as the Planck scale in the deep UV.

hep-ph↗

arXiv2020

Pair production of dark particles in meson decays

Rare decays of $K$ and $B$ mesons provide a powerful probe of dark sectors with light new particles. We show that the pair production of $O(100\,{\rm MeV})$ dark states can be probed with the decays of $K_L$ mesons, owing to the enhanced two-body kinematics, $K_L\to X_1X_2$ or $X_2X_2$. If either or these two particles is unstable, e.g. $X_2\to X_1π^0$, $X_2\to X_1γ$ or $X_{1,2}\to γγ$, such decays could easily mimic $K_L\to π^0 ν\overlineν$ signatures, while not being ruled out by the decays of charged kaons. We construct explicit models that have enhanced $K_L$ decay signatures, and are constrained by the results of the KOTO experiment. We note that recently reported excess events can also be accommodated while satisfying all other constraints ($B$ decays, colliders, beam dumps). These models are based on the extensions of the gauge and/or scalar sector of the theory. The lightest of $X_{1,2}$ particles, if stable, could constitute the entirety of dark matter.

hep-ph↗

arXiv2020

Reheating and Post-inflationary Production of Dark Matter

We perform a systematic analysis of dark matter production during post-inflationary reheating. Following the period of exponential expansion, the inflaton begins a period of damped oscillations as it decays. These oscillations and the evolution of temperature of the thermalized decay products depend on the shape of the inflaton potential $V(Φ)$. We consider potentials of the form $Φ^k$. Standard matter-dominated oscillations occur for $k=2$. In general, the production of dark matter may depend on either (or both) the maximum temperature after inflation, or the reheating temperature, where the latter is defined when the Universe becomes radiation dominated. We show that dark matter production is sensitive to the inflaton potential and depends heavily on the maximum temperature when $k>2$. We also consider the production of dark matter with masses larger than the reheating temperature.

hep-ph↗

arXiv2019

Inflation and Leptogenesis in High-Scale Supersymmetry

No-scale supergravity provides a successful framework for Starobinsky-like inflation models. Two classes of models can be distinguished depending on the identification of the inflaton with the volume modulus, $T$ (C-models), or a matter-like field, $ϕ$ (WZ-models). When supersymmetry is broken, the inflationary potential may be perturbed, placing restrictions on the form and scale of the supersymmetry breaking sector. We consider both types of inflationary models in the context of high-scale supersymmetry. We further distinguish between models in which the gravitino mass is below and above the inflationary scale. We examine the mass spectra of the inflationary sector. We also consider in detail mechanisms for leptogenesis for each model when a right-handed neutrino sector, used in the seesaw mechanism to generate neutrino masses, is employed. In the case of C-models, reheating occurs via inflaton decay to two Higgs bosons. However, there is a direct decay channel to the lightest right-handed neutrino which leads to non-thermal leptogenesis. In the case of WZ-models, in order to achieve reheating, we associate the matter-like inflaton with one of the right-handed sneutrinos whose decay to the lightest right handed neutrino simultaneously reheats the Universe and generates the baryon asymmetry through leptogenesis.

hep-ph↗

arXiv2019

Limits on $R$-parity Violation in High Scale Supersymmetry

We revisit the limits on $R$-parity violation in the minimal supersymmetric standard model. In particular, we focus on the high-scale supersymmetry scenario in which all the sparticles are in excess of the inflationary scale of approximately $10^{13}$ GeV, and thus no sparticles ever come into thermal equilibrium. In this case the cosmological limits, stemming from the preservation of the baryon asymmetry that have been previously applied for weak scale supersymmetry, are now relaxed. We argue that even when sparticles are never in equilibrium, $R$-parity violation is still constrained via higher dimensional operators by neutrino and nucleon experiments and/or insisting on the preservation of a non-zero $B-L$ asymmetry.

hep-ph↗

arXiv2019

Hillclimbing inflation in metric and Palatini formulations

A new setup of cosmic inflation with a periodic inflaton potential and conformal factor is discussed in the metric and Palatini formulations of gravity. As a concrete example, we focus on a natural-inflation-like inflaton potential, and show that the inflationary predictions fall into the allowed region of cosmic microwave background observations in both formulations.

gr-qc↗

arXiv2019

Radiative Production of Non-thermal Dark Matter

We compare dark matter production from the thermal bath in the early universe with its direct production through the decay of the inflaton. We show that even if dark matter does not possess a direct coupling with the inflaton, Standard Model loop processes may be sufficient to generate the correct relic abundance.

hep-ph↗