arXiv subjects

Kentaro Kasai

Publications and source records attributed to Kentaro Kasai.

10 recordsLinked to original sources

arXiv2026

Merger rate of initially clustered primordial black holes for the two-body channel

Primordial black holes (PBHs) may form an initially clustered population depending on their production mechanism. Motivated by binary black-hole merger events observed by gravitational-wave interferometers, we revisit the evaluation of the merger rate of PBH binaries and extend the formalism to include the effects of clustering. We show that, in the presence of relatively weak PBH clustering, the LIGO-Virgo-KAGRA events can be explained with a smaller value of $f_{\mathrm{PBH}}$ than in scenarios with Poisson-distributed PBHs, at least in the early two-body channel. However, for stronger clustering, the merger rate in the two-body channel is significantly suppressed due to the formation of three-body systems.

astro-ph.CO↗

arXiv2026

Microlensing events and primordial black holes in the axionlike curvaton model

Recently, Subaru Hyper Suprime-Cam (HSC) observations found 12 candidates for microlensing events. These events can be explained by primordial black holes (PBHs) with masses of $10^{-7}$-$10^{-6} M_\odot$ and a fraction of all dark matter of $f_\mathrm{PBH} = \mathcal{O}(10^{-1})$. In this paper, we consider the PBH production in two types of the axionlike curvaton models, which predict an enhancement of the curvature perturbations on small scales. We show that the microlensing events can be explained in the axionlike curvaton model and discuss the cosmological implications such as gravitational waves.

astro-ph.CO↗

arXiv2025

Primordial black hole formation from the merger of oscillons

We show that the merger of oscillons results in a broad spectrum of the oscillon mass. A huge number of oscillon samples obtained from numerical lattice simulations reveal that the oscillon mass distribution has an exponential tail in a heavy-mass region. This enables us to infer the fractional abundance of heavy oscillons. Using the criterion for the primordial black hole (PBH) formation from the oscillon collapse obtained in previous studies, we estimate the abundance of PBHs and conclude that a sizable number of PBHs can be produced from oscillons. It can be an alternative PBH formation mechanism without employing the tuning of the inflaton potential to enhance the small-scale density fluctuations in the conventional PBH formation scenario.

astro-ph.CO↗

arXiv2025

Resonant production of sterile neutrino dark matter with a refined numerical scheme

The existence of a large primordial neutrino asymmetry is an intriguing possibility, both observationally and theoretically. Such an asymmetry can lead to the resonant production of $\mathrm{keV}$-scale sterile neutrinos, which are a fascinating candidate for dark matter. In this paper, we comprehensively revisit the resonant production processes with a refined numerical analysis, adopting a dynamical discretization of momentum modes to take care of the sharpness of the resonance. We find parameter regions consistent with X-ray and Lyman-$\alpha$ constraints for lepton-to-entropy ratio $\gtrsim \mathcal{O}(10^{-3})$ and $m_{\nu_s}\gtrsim 20\,$keV. We also explore the Affleck-Dine mechanism as a possible origin for such asymmetries. While previous studies considered resonant production after lepton number generation, we numerically investigate cases where a fraction of sterile neutrinos is produced during lepton number injection. In this regime, some parameter sets can shorten the free-streaming length and reduce the required mixing angle to match the observed dark matter abundance, thereby mitigating the observational constraints.

hep-ph↗

arXiv2024

Gravitational Waves from Particles Produced from Bubble Collisions in First-Order Phase Transitions

We discuss a new source of gravitational waves (GWs) from first-order phase transitions. The collisions of bubbles of the new phase can efficiently produce particles that couple to the background field undergoing the transition, thereby transferring a significant fraction of the released vacuum energy into a distribution of inhomogeneous and dynamic particle populations that persist long after the bubbles have disappeared. We study the GWs produced by such particle distributions, showing that GWs arise from the quadrupolar anisotropy in the radiation emitted from the bubble collisions, and present a semi-analytical calculation of the two-point correlation function for the associated energy distributions. We find that this new contribution can qualitatively modify the overall GW signal from such phase transitions, creating a distinct shift in the spectral slope at low frequencies that could be observed by future GW experiments. It is therefore important to take this new contribution into account for any transition where the background field has significant self-coupling or couplings to other fields that could lead to efficient particle production at bubble collision.

astro-ph.CO↗

arXiv2024

Affleck-Dine leptogenesis scenario for resonant production of sterile neutrino dark matter

Sterile neutrino is a fascinating candidate for dark matter. In this paper, we examine the Affleck-Dine (AD) leptogenesis scenario generating a large lepton asymmetry, which can induce the resonant production of sterile neutrino dark matter via the Shi-Fuller (SF) mechanism. We also revisit the numerical calculation of the SF mechanism and the constraints from current X-ray and Lyman-$α$ forest observations. We find that the AD leptogenesis scenario can explain the production of sterile neutrino dark matter by incorporating a non-topological soliton with a lepton charge called L-ball. Finally, we discuss an enhancement of second-order gravitational waves at the L-ball decay and investigate the testability of our scenario with future gravitational wave observations.

hep-ph↗

arXiv2024

Supermassive black hole formation from Affleck-Dine mechanism with suppressed clustering on large scales

We study a primordial black hole (PBH) formation model based on the framework of the inhomogeneous Affleck-Dine (AD) mechanism, which can explain the seeds of supermassive black holes (SMBHs). This model, however, predicts strong clustering of SMBHs that is inconsistent with the observation of angular correlation of quasars. In this paper, we propose a modified model that can significantly reduce the PBH clustering on large scales by considering a time-dependent Hubble-induced mass during inflation. The quasar angular correlation is suppressed by the large Hubble-induced mass in the early stage of inflation while the small Hubble-induced mass in the late stage leads to the AD field fluctuations large enough for PBH formation as in the original model. As a result, the modified scenario can successfully explain the seeds of SMBHs.

astro-ph.CO↗

arXiv2024

Revisiting the Affleck-Dine mechanism for primordial black hole formation

We study a primordial black hole (PBH) formation scenario based on the Affleck-Dine (AD) mechanism and investigate two PBH mass regions: $M \sim 30 M_\odot$ motivated by the LIGO-Virgo observations of the binary black hole mergers and $M \gtrsim 10^4 M_\odot$ motivated by the observations of supermassive black holes at the center of galaxies. In the previous studies, it has been considered that the inhomogeneous AD baryogenesis generates regions with a large baryon asymmetry, some of which collapse into PBHs. In this paper, we show that this scenario is severely constrained due to the baryon asymmetry remaining outside PBHs, which would spoil the success of the big bang nucleosynthesis. Then, we propose an alternative scenario where the AD leptogenesis results in the inhomogeneous formation of Q-balls with lepton charges, which collapse into PBHs. As a result, we find that our scenario can explain the favorable PBH abundance without conflicting with the observational constraints.

astro-ph.CO↗

arXiv2023

Primordial Origin of Supermassive Black Holes from Axion Bubbles

We study a modification of the primordial black hole (PBH) formation model from axion bubbles. We assume that the Peccei-Quinn scalar rolls down in the radial direction from a large field value to the potential minimum during inflation, which suppresses the axion fluctuations and weakens the clustering of PBHs on large scales. We find that the modified model can produce a sufficient number of PBHs that seed the supermassive black holes while avoiding the observational constraints from isocurvature perturbations and angular correlation of the high-redshift quasars.

astro-ph.CO↗

arXiv2023

Clustering of Primordial Black Holes from QCD Axion Bubbles

We study the clustering of primordial black holes (PBHs) and axion miniclusters produced in the model proposed to explain the LIGO/Virgo events or the seeds of the supermassive black holes (SMBHs) in arXiv:2006.13137. It is found that this model predicts large isocurvature perturbations due to the clustering of PBHs and axion miniclusters, from which we obtain stringent constraints on the model parameters. Specifically, for the axion decay constant $f_a=10^{16}~\mathrm{GeV}$, which potentially accounts for the seeds of the SMBHs, the PBH fraction in dark matter should be $f_\mathrm{PBH}\lesssim7\times 10^{-10}$. Assuming that the mass of PBHs increases by more than a factor of $\mathcal{O}(10)$ due to accretion, this is consistent with the observed abundance of SMBHs. On the other hand, for $f_a=10^{17}~\mathrm{GeV}$ required to produce PBHs of masses detected in the LIGO/Virgo, the PBH fraction should be $f_\mathrm{PBH}\lesssim6\times 10^{-8}$, which may be too small to explain the LIGO/Virgo events, although there is a significant uncertainty in calculating the merger rate in the presence of clustering.

astro-ph.CO↗