arXiv subjects

Soichiro Hashiba

Publications and source records attributed to Soichiro Hashiba.

9 recordsLinked to original sources

arXiv2022

Generation of neutrino dark matter, baryon asymmetry, and radiation after quintessential inflation

We construct a model explaining dark matter, baryon asymmetry and reheating in quintessential inflation model. Three generations of right-handed neutrinos having hierarchical masses, and the light scalar field leading to self-interaction of active neutrinos are introduced. The lightest sterile neutrino is a dark matter candidate produced by a Dodelson-Widrow mechanism in the presence of a new light scalar field, while the heaviest and the next heaviest sterile neutrinos produced by gravitational particle production are responsible for the generation of the baryon asymmetry. Reheating is realized by spinodal instabilities of the Standard Model Higgs field induced by the non-minimal coupling to the scalar curvature, which can solve overproduction of gravitons and curvature perturbation created by the Higgs condensation.

hep-ph↗

arXiv2022

An analytic evaluation of gravitational particle production of fermions via Stokes phenomenon

The phenomenon of gravitational particle production can take place for quantum fields in curved spacetime. The abundance and energy spectrum of gravitationally produced particles is typically calculated by solving the field's mode equations on a time-dependent background metric. For purposes of studying dark matter production in an inflationary cosmology, these mode equations are often solved numerically, which is computationally intensive, especially for the rapidly-oscillating high-momentum modes. However, these same modes are amenable to analytic evaluation via the Exact Wentzel-Kramers-Brillouin (EWKB) method, where gravitational particle production is a manifestation of the Stokes phenomenon. These analytic techniques have been used in the past to study gravitational particle production for spin-0 bosons. We extend the earlier work to study gravitational production of spin-1/2 and spin-3/2 fermions. We derive an analytic expression for the connection matrix (valid to all orders in perturbations) that relates Bogoliubov coefficients across a Stokes line connecting a merged pair of simple turning points. By comparing the analytic approximation with a direct numerical integration of the mode equations, we demonstrate an excellent agreement and highlight the utility of the Stokes phenomenon formalism applied to fermions. We discuss the implications for an analytic understanding of catastrophic particle production due to vanishing sound speed, which can occur for a spin-3/2 Rarita-Schwinger field.

hep-th↗

arXiv2021

Gauge Field Production and Schwinger Reheating in Runaway Axion Inflation

In a class of (pseudoscalar) inflation, inflationary phase is followed by a kination phase, where the Universe is dominated by the kinetic energy of the inflaton that runs away in a vanishing scalar potential. In this class of postinflationary evolution of the Universe, reheating of the Universe cannot be achieved by the inflaton particle decay, which requires its coherent oscillation in a quadratic potential. In this study, we explore the U(1) gauge field production through the Chern-Simons coupling between the pseudoscalar inflaton and the gauge field during the kination era and examine the subsequent pair-particle production induced by the amplified gauge field known as the Schwinger effect, which can lead to reheating of the Universe. We find that with a rough estimate of the Schwinger effect for the Standard Model hyper U(1) gauge field and subsequent thermalization of the pair-produced particles, a successful reheating of the Universe can be achieved by their eventual domination over the kinetic energy of the inflaton, with some reasonable parameter sets. This can be understood as a concrete realization of the "Schwinger reheating". Constraints from the later-time cosmology are also discussed.

hep-ph↗

arXiv2021

Stokes phenomenon and gravitational particle production -- How to evaluate it in practice

We revisit gravitational particle production from the Stokes phenomenon viewpoint, which helps us make a systematic way to understand asymptotic behavior of mode functions in time-dependent background. One of our purposes of this work is to make the method more practical for evaluation of non-perturbative particle production rate. In particular, with several examples of time-dependent backgrounds, we introduce some approximation methods that make the analysis more practical. Specifically, we consider particle production in simple expanding backgrounds, preheating after $R^2$ inflation, and a transition model with smoothly changing mass. As we find several technical issues in analyzing the Stokes phenomenon of each example, we discuss how to simplify the problems while showing the accuracy of analytic estimation under the approximations we make.

hep-th↗

arXiv2021

Particle production induced by vacuum decay in real time dynamics

We discuss particle production associated with vacuum decay, which changes the mass of a scalar field coupled to a background field which induces the decay. By utilizing the Stokes phenomenon, we can optimally track the time-evolution of mode function and hence calculate particle production properly. In particular, we use real time formalisms for vacuum decay in Minkowski and de Sitter spacetime together with the Stokes phenomenon method. For each case, we consider the flyover vacuum decay model and stochastic inflation, respectively. Within the real time formalism, the particle production can be viewed as that caused by nontrivial external fields. This gives us a novel perspective of the real time formalism of vacuum decay.

hep-th↗

arXiv2020

Artifact-less Coded Aperture Imaging in the X-ray Band with Multiple Different Random Patterns

The coded aperture imaging technique is a useful method of X-ray imaging in observational astrophysics. However, the presence of imaging noise or so-called artifacts in a decoded image is a drawback of this method. We propose a new coded aperture imaging method using multiple different random patterns for significantly reducing the image artifacts. This aperture mask contains multiple different patterns each of which generates a different artifact distribution in its decoded image. By summing all decoded images of the different patterns, the artifact distributions are cancelled out, and we obtain a remarkably accurate image. We demonstrate this concept with imaging experiments of a monochromatic 16 keV hard X-ray beam at the synchrotron photon facility SPring-8, using the combination of a CMOS image sensor and an aperture mask that has four different random patterns composed of holes with a diameter of 27 um and a separation of 39 um. The entire imaging system is installed in a 25 cm-long compact size, and achieves an angular resolution of < 30'' (full width at half maximum). In addition, we show by Monte Carlo simulation that the artifacts can be reduced more effectively if the number of different patterns increases to 8 or 16.

astro-ph.IM↗

arXiv2019

Dark matter and baryon-number generation in quintessential inflation via hierarchical right-handed neutrinos

Incorporating three generations of right-handed Majorana neutrinos to quintessential inflation, we construct a model which simultaneously explains inflation, dark energy, dark matter and baryogenesis. These neutrinos have hierarchical masses $M_3 \sim 10^{13}$GeV, $M_2 \sim 10^{11}$GeV, $M_1 \sim 10$keV and are produced by gravitational particle production in the kination regime after inflation. The heaviest, the intermediate, and the lightest account for reheating, CP violation of leptogenesis, and dark matter, respectively. This model can be tested in various ways with forthcoming observations.

hep-ph↗

arXiv2019

Gravitational particle creation for dark matter and reheating

The purely gravitational dark matter (PGDM) which interacts with the standard model particles only by gravitational interaction has recently been discussed. Due to its feeble interaction, PGDM may be produced mainly by the gravitational particle creation, which plays an important role in the reheating after kinetically driven inflation and some potential-driven inflation without subsequent inflaton oscillating phase. Therefore, we consider the possibility of the gravitational reheating model which can also explain the present PGDM density at the same time. We consider a model where two massive scalar fields are incorporated into the standard model besides the inflation sector. We show that the gravitational particle creation prevails over the thermal creation --- the freeze-in process --- and it can actually explain the reheating and the present abundance of dark matter if one of the scalar particles is as heavy as the Hubble parameter during inflation $\sim10^{13}$ GeV and finally decays into radiation via sufficiently weak coupling, and the other is a stable PGDM with its mass of the order of $10^3$ GeV.

hep-ph↗

arXiv2019

Gravitational reheating through conformally coupled superheavy scalar particles

We calculate the number density and the energy density of a massive scalar particle conformally coupled to gravity produced by gravitational particle creation in the case the kination stage follows inflation. In this model, mode functions are derived in terms of exact or numerical solutions of the equation of motion without using the adiabatic approximation. We define the adiabatic vacuum in each stage and calculate the produced number density and the energy density. The resultant power spectra show that even the superheavy particle which is as heavy as or much heavier than the Hubble scale during inflation can be produced abundantly if the transition time scale from inflation to the kination stage is smaller than the inverse of the mass of the scalar field. We also give simple forms of the reheating temperature for two cases, one that the produced scalar particles decay instantly and the other that they decay when their energy density exceeds that of inflaton.

gr-qc↗