arXiv subjects

Shinta Kasuya

Publications and source records attributed to Shinta Kasuya.

At least 19 recordsLinked to original sources

arXiv2026

Parameter estimation from the transit light curve including gravitational lensing effects

We study transit light curves of exoplanets, incorporating gravitational lensing effects, and explore the possibility of estimating planetary masses using only the transit light curve. We analyze existing data for planets with relatively large orbital distances ($>1$ AU) from Kepler and TESS to evaluate how well their masses can be constrained. For six exoplanets, we derive 2$\sigma$ upper limits of 200-2000 Jupiter masses. We also construct mock data to assess the feasibility of determining masses for exoplanets with larger orbital radii than currently known. Our results show that a precision of $3\times 10^{-6}$ is required to recover the true masses of planets with 10 (5) Jupiter masses at orbital radii of $a=20$ (100) AU. Furthermore, we demonstrate that neglecting gravitational effects can lead to underestimation of exoplanet radii, particularly for planets with large orbital separations. Using mock data with a precision of $2\times 10^{-4}$, assuming a 30 minutes cadence with Kepler for a $K_p=14$ mag star, we find that the estimated radii can be underestimated by 1-20% for 5 and 10 Jupiter-mass planets with $a=$ 10-100 AU. These results highlight the importance of including microlensing effects when modeling transit light curves of wide-orbit planets.

astro-ph.EP↗

arXiv2025

Formation of primordial black holes through Q-balls

We study the primordial black hole (PBH) formation from Q-balls that are non-topological solitons in scalar field theories. We develop a formula for calculating the density perturbations from the Q-ball charge distribution. We also re-examine the condition for the PBH formation in the matter-dominated era and show that the previously derived formula for super-horizon density fluctuations can be applied to the sub-horizon density perturbations. As an example, we consider the Q-balls in the case of gauge-mediated supersymmetry (SUSY) breaking, whose charge distribution was obtained by the lattice simulation. We find that the density perturbations are large enough to produce a significant number of PBHs with mass $10^{-15}\,M_\odot -5\times 10^{-12}\, M_\odot$, which can explain all the dark matter in the universe. In the context of supersymmetry, this mass range corresponds to the SUSY breaking scale $\sim 10^6$ GeV, which is consistent with the SUSY particle masses $\sim 10$ TeV.

hep-ph↗

arXiv2025

Charge distribution of the gauge-mediation type Q ball

We numerically study the formation of the gauge-mediation type Q balls in the logarithmic square potential on three-dimensional lattices. We obtain the broad charge distribution of the Q ball of this type for the first time, to the best of our knowledge. The charge of the Q ball at the peak of the distribution is smaller than what we estimated as the average of the largest tens of the Q balls in the logarithmic potential for the same initial amplitude of the field at the onset of its oscillation. We also discuss some impacts of the broad distribution on cosmology and astrophysics. In the B ball (Q being the baryon number) case, the broad distribution would lead to the coexistence of both stable and unstable B balls. We find that stable B balls can account for the dark matter of the universe without affecting successful big bang nucleosynthesis by the decay of the unstable B balls, but the baryon number of the universe cannot be explained by them. On the other hand, the large L balls (Q being the lepton number) would be the dark matter as well while avoiding the constraints on the X and/or gamma rays from the decay of the smaller L balls.

hep-ph↗

arXiv2025

Detection of molecular clouds in the PeVatron candidate source LHAASO J0341$+$5258 by the Nobeyama 45-m radio telescope

We report a new CO observation survey of LHAASO J0341$+$5258 using the Nobeyama Radio Observatory (NRO) 45-m telescope. LHAASO J0341$+$5258 is one of the unidentified ultra-high-energy (UHE; E $>$100 TeV) gamma-ray sources detected by LHAASO. Our CO observations were conducted in February and March 2024, with a total observation time of 36 hours, covering the LHAASO source ($\sim$0.3-0.5 degrees in radius) and its surrounding area (1$\times$1.5 degrees). Within the LHAASO source extent, we identified five compact ($<$ 2 pc) molecular clouds at nearby distances ($<$ 1-4 kpc). These clouds can serve as proton-proton collision targets, producing hadronic gamma rays via neutral pion decays. Based on the hydrogen densities (700-5000 cm$^{-3}$) estimated from our CO observations and archived HI data from the DRAO survey, we derived the total proton energy of $W_p$ (E $>$ 1 TeV) $\sim$ 10$^{45}$ erg to account for the gamma-ray flux. One of the molecular clouds appears to be likely associated with an asymptotic giant branch (AGB) star with an extended CO tail, which may indicate some particle acceleration activities. However, the estimated maximum particle energy below 100 TeV makes the AGB-like star unlikely to be a PeVatron site. We conclude that the UHE emission observed in LHAASO J0341$+$5258 could be due to hadronic interactions between the newly discovered molecular clouds and TeV-PeV protons originating from a distant SNR or due to leptonic emission from a pulsar wind nebula candidate, which is reported in our companion X-ray observation paper (DiKerby et al. 2025).

astro-ph.HE↗

arXiv2024

MeV gamma rays from Q-ball decay

We study the supersymmetric Q balls which decay at present and find that they create a distinctive spectrum of gamma rays at around O(10) MeV. The charge of the Q ball is lepton numbers in order for the lifetime to be as long as the present age of the universe, and the main decay products are light leptons. However, as the charge of the Q ball decreases, the decay channel into pions becomes kinematically allowed towards the end of the decay, and the pions are produced at rest. Immediately, $\pi^0$ decays into two photons with the energy of 67.5 MeV, half the pion mass, which exhibits a unique emission line. In addition, $\pi^\pm$ decay into $\mu^\pm$, which further decay with emitting internal bremsstrahlung, whose spectrum has a sharp cutoff at $\sim$50 MeV. If the observations would find these peculiar features of the gamma-ray spectrum in the future, it could be a smoking gun of the supersymmetric Q-ball decay at present.

hep-ph↗

arXiv2022

Enhancement of second-order gravitational waves at Q-ball decay

The recent observation of $^4$He favors a large lepton asymmetry at the big bang nucleosynthesis. If Q-balls with a lepton charge decay after the electroweak phase transition, such a large lepton asymmetry can be generated without producing too large baryon asymmetry. In this scenario, Q-balls dominate the universe before the decay and induces the sharp transition from the early matter-dominated era to the radiation-dominated era. In this transition, the gravitational waves (GWs) are enhanced through a second-order effect of the scalar perturbations. We evaluate the density of the produced GWs and show that pulsar timing array observations can probe this scenario depending on the amplitude of the scalar perturbations.

astro-ph.CO↗

arXiv2021

Throat effects on shadows of Kerr-like wormholes

We revisit to investigate shadows cast by Kerr-like wormholes. The boundary of the shadow is determined by unstable circular photon orbits. We find that, in certain parameter regions, the orbit is located at the throat of the Kerr-like wormhole, which was not considered in the literature. In these cases, the existence of the throat alters the shape of the shadow significantly, and makes it possible for us to differentiate it from that of a Kerr black hole.

gr-qc↗

arXiv2020

Revisiting oscillon formation in KKLT scenario

KKLT scenario has succeeded in stabilizing the volume modulus and constructing metastable de Sitter (dS) vacua in type IIB string theory. We revisit to investigate the possibility of the oscillon (or I-ball) formation in the KKLT scenario when the volume modulus is initially displaced from the dS minimum. Special attention is paid to physically realistic initial conditions of the volume modulus, which was not taken in the literature. Using lattice simulations, we find that oscillons do not form unless the volume modulus is initially placed at very near the local maximum, which requires severe fine-tuning.

hep-ph↗

arXiv2019

Novel Flavon Stabilization with Trimaximal Neutrino Mixing

We construct a supersymmetric $S_4$ flavor symmetry model with one of the trimaximal neutrino mixing patterns, the so-called TM$_1$, by using the novel way to stabilize flavons, which we proposed recently. The flavons are assumed to have tachyonic supersymmetry breaking mass terms and stabilized by higher-dimensional terms in the potential. We can obtain the desired alignment structure of the flavon vacuum expectation values to realize neutrino masses and mixings consistent with the current observations. This mechanism naturally avoids the appearance of dangerous cosmological domain walls.

hep-ph↗

arXiv2018

Flavon Stabilization in Models with Discrete Flavor Symmetry

We propose a simple mechanism for stabilizing flavon fields with aligned vacuum structure in models with discrete flavor symmetry. The basic idea is that flavons are stabilized by the balance between the negative soft mass and non-renormalizable terms in the potential. We explicitly discuss how our mechanism works in $A_4$ flavor model, and show that the field content is significantly simplified. It also works as a natural solution to the cosmological domain wall problem.

hep-ph↗

arXiv2018

Quadratic chaotic inflation with a logarithmic-corrected mass

We consider a simple modification of quadratic chaotic inflation. We add a logarithmic correction to the mass term, and find that this model can be consistent with the latest cosmological observations such as the Planck 2018 data, in combination with the BICEP2/Keck Array and the baryon acoustic oscillation data. Since the model predicts the lower limit for the tensor-to-scalar ratio r for the present allowed values of the spectral index n_s, it could be tested by the cosmic microwave background polarization observation in the near future. In addition, we consider higher-order logarithmic corrections. Interestingly, we observe that the scalar spectral index n_s and r stay in rather a narrow region of the parameter space. Moreover, they reside in a completely different region from that for the logarithmic corrections to the quartic coupling. Therefore, future observations may distinguish which kind of corrections should be included, or even single out the form of the interactions.

astro-ph.CO↗

arXiv2016

Influence on Starobinsky inflation by other fields with large amplitude

The Starobinsky model is one of the inflation models consistent with the result of CMB observation by the Planck satellite. We consider the dynamics of the Starobinsky inflation in the presence of another scalar field with a large expectation value during inflation due to a negative Hubble-induced mass. We find that it would be affected if the other field has an amplitude close to the Planck scale. In this case, we may observe such effects on the Starobinsky model by future CMB experiments.

hep-ph↗

arXiv2015

Axino dark matter and baryon number asymmetry production by the Q-ball decay in gauge mediation

We investigate the Q-ball decay into the axino dark matter in the gauge-mediated supersymmetry breaking. In our scenario, the Q ball decays mainly into nucleons and partially into axinos to account respectively for the baryon asymmetry and the dark matter of the universe. The Q ball decays well before the big bang nucleosynthesis so that it is not affected by the decay. We show the region of the parameters which realizes this scenario.

hep-ph↗

arXiv2015

IceCube potential for detecting Q-ball dark matter in gauge mediation

We study Q-ball dark matter in gauge-mediated supersymmetry breaking, and seek the possibility of detection in the IceCube experiment. We find that the Q balls would be the dark matter in the parameter region different from that for gravitino dark matter. In particular, the Q ball is a good dark matter candidate for low reheating temperature, which may be suitable for the Affleck-Dine baryogenesis and/or nonthermal leptogenesis. Dark matter Q balls are detectable by IceCube-like experiments in the future, which is the peculiar feature compared to the case of gravitino dark matter.

hep-ph↗

arXiv2014

Q-ball dark matter and baryogenesis in high-scale inflation

We investigate the scenario that one flat direction creates baryon asymmetry of the unverse, while Q balls from another direction can be the dark matter in the gauge-mediated supersymmetry breaking for high-scale inflation. Isocurvature fluctuations are suppressed by the fact that the Affleck-Dine field stays at around the Planck scale during inflation. We find that the dark matter Q balls can be detected in IceCube-like experiments in the future.

hep-ph↗

arXiv2014

Flat Direction Inflation with Running Kinetic Term and Baryogenesis

We consider a possibility that one of the flat directions in the minimal supersymmetric standard model plays the role of the inflaton field and realizes large-field inflation. This is achieved by introducing a generalized shift symmetry on the flat direction, which enables us to control the inflaton potential over large field values. After inflation, higher order terms allowed by the generalized shift symmetry automatically cause a helical motion of the field to create the baryon number of the universe, while baryonic isocurvature fluctuations are suppressed.

hep-ph↗

arXiv2014

Baryogenesis from the Gauge-mediation type Q ball and the New type of Q ball as dark matter

We investigate that the two types of the Q balls explain the baryon asymmetry and the dark matter of the universe in the gauge-mediated supersymmetry breaking. The gauge-mediation type Q balls of one flat direction produce baryon asymmetry, while the new type Q balls of another flat direction become the dark matter. We show that the dark matter new type Q balls are free from the neutron star constraint. n=5 gauge mediation type and n=6 new type Q balls are displayed as an example, where the potential is lifted by the superpotential Phi^n. These dark matter Q balls may be detected by future observations, such as in advanced IceCube-like observations.

hep-ph↗

arXiv2012

Revisiting the gravitino dark matter and baryon asymmetry from Q-ball decay in gauge mediation

We reinvestigate the scenario that the amount of the baryons and the gravitino dark matter is naturally explained by the decay of the Q balls in the gauge-mediated SUSY breaking. Equipped by the more correct decay rates into gravitinos and baryons recently derived, we find that the scenario with the direct production of the gravitino dark matter from the Q-ball decay works naturally.

hep-ph↗