arXiv subjects

Nobuyuki Sawado

Publications and source records attributed to Nobuyuki Sawado.

At least 19 recordsLinked to original sources

arXiv2026

Soliton solution of a gravitating chiral-quark soliton model: dynamical fermion and Dirac-sea in general relativity

In this paper, we study Einstein-Dirac system based on a Dirac fermion coupled with a nonlinear chiral field on static spherically symmetric spacetime. Gravitational effects on the dynamical mass of fermions are examined. The chiral-quark soliton model (CQSM) originally was a model of hadron inspired by the low-energy regime of large-$N_\textrm{c}$ QCD, realizing localized fermions with the full inclusion of the Dirac-sea, which is derived from a regularized one-fermion loop. We extend the CQSM in spherically symmetric curved spacetime and coupled with the Einstein gravity. We successfully solve the CQSM and the Einstein equation self-consistently, and obtain the spectral flow of the fermion energies and also of the ADM mass. The Dirac-sea dilutes the effect of the energy-momentum tensor, which induces an inactive point for the impact of gravity even when the localizing matter exists. The Dirac-sea effect becomes dominant for larger dynamical mass $M\sim \langle\barψψ\rangle$, leading to the emergence of the negative ADM mass.

gr-qc↗

arXiv2026

A Modified Center-of-Mass Conservation Law in Finite-Domain Simulations of the Zakharov--Kuznetsov Equation

We investigate conservation laws of the two-dimensional Zakharov--Kuznetsov (ZK) equation, a natural higher-dimensional and non-integrable extension of the Korteweg--de Vries equation. The ZK equation admits three scalar conserved quantities -- mass, momentum, and energy -- represented as $I_1$, $I_2$, and $I_3$, as well as a vector-valued quantity $\bm{I}_4$. In high-accuracy numerical simulations on a finite double-periodic domain, most of these quantities are well preserved, while a systematic temporal drift is observed only in the $x$-component $I_{4x}$. We show that the nontrivial evolution of $I_{4x}$ originates from an explicit boundary-flux contribution, which is induced by fluctuations of the solution and its spatial derivatives at the domain boundaries. We successfully identify the source of the inaccuracy in the numerical solutions. Motivated by this analysis, we define a modified center-of-mass quantity $I_{4x}^{\mathrm{mod}}$ and demonstrate its conservation numerically for single-pulse configurations. The modified quantity thus provides a consistent conservation law for the ZK equation and yields an appropriate description of center-of-mass motion in finite-domain numerical simulations.

nlin.SI↗

arXiv2026

Topological Charge Asymmetry in a $\mathbb{C}\mathrm{P}^N$ Skyrmion-Fermion Coupled System

Topology plays a central role in classifying solitonic configurations in field theories, providing robustness and a nonperturbative label, the so-called topological charge $Q$. In soliton-fermion coupled systems, the relation between the topological charge and the number of zero modes is well established through the index theorem. However, the physical consequences of the sign of the topological charge have remained largely unexplored. In this work, we study fermions in $2+1$ dimensions coupled to Skyrmions with target space $\mathbb{C}\mathrm{P}^N$, particularly focusing on the backreactions of the fermions and on the sign of the topological charge. We obtain the solutions in a self-consistent manner, which exhibit an asymmetry with respect to the topological charge $\pm Q$ especially in the strong coupling regimes. This asymmetry is caused from the fermionic eigenvalue problem inherent in the self-consistent formulation. Although the Lagrangian is symmetric under $Q\to-Q$, the coupled equations for the Skyrmions and anti-Skyrmions become inequivalent once fermionic backreaction is taken into account. We demonstrate the mechanism in $\mathbb{C}\mathrm{P}^1$ and $\mathbb{C}\mathrm{P}^2$ Skyrmions, but the analysis is directly extendable for the general $\mathbb{C}\mathrm{P}^N$.

hep-th↗

arXiv2025

$\mathbb{C}\mathrm{P}^2$ Skyrmion with Fermion Backreaction

When fermions interact with a topological soliton, they localize on the soliton. In studies of such systems, solitons are often treated as fixed background fields, and the backreaction due to the fermion localization is usually neglected for simplicity. In this work, we investigate the backreaction of localized fermions on a $\mathbb{C}\mathrm{P}^2$ Skyrmion. We find that the Skyrmion profile deforms in response, becoming more concentrated around the localized fermions. We also discuss the possibility that this backreaction may play a role analogous to that of the potential term.

hep-th↗

arXiv2025

Multishell Dirac fermions in the Einstein-Dirac system

We present multifermions in the spherically symmetric Einstein-Dirac system. Dirac fermions are self-localized within a spherically symmetric Einstein gravity, i.e., the Schwarzschild-like space-time metric. Most of previous studies of the Einstein-Dirac system are restricted to two neutral fermions or to many fermions with the same high-angular momentum filling a single shell. Our model considers full-filling of fermions in multiple shells, similarly to the conventional nuclear shell model. We solve the model for fermion numbers $N_\textrm{f}=2,6,12$ and $20$, which can realize a spherically symmetric system. Even single-shell multifermions exhibit a multipeak structure and fragmentation in the high redshift region. The behavior observed in our multishell model can be explained by interactions between the shells and resulting delocalization. We also investigate the pressure of the solutions which defines the existence (or absence) of intershell interactions. The radial pressure is attractive, supporting compactness of the solutions. Finally, we show the correlations between the nontrivial changes in the Shannon entropy (a logarithmic measure of information content) and the structural deformations of the solutions.

gr-qc↗

arXiv2024

Spectral flow of fermions in the $\CP^2$ (anti-)instanton, and the sphaleron with vanishing topological charge

The spectral flow is ubiquitous in the physics of soliton-fermion interacting systems. We study the spectral flows related to a continuous deformation of background soliton solutions, which enable us to develop insight into the emergence of fermionic zero modes and the localization mechanism of fermion densities. We investigate a $\CP^2$ nonlinear sigma model in which there are the (anti-) instantons and also the sphalerons with vanishing topological charge. The standard Yukawa coupling of the fermion successfully generates infinite towers of the spectra and the spectral flow is observed when increasing the size of such solitons. At that moment, the localization of the fermions on the solitons emerges. The avoided crossings are also observed in several stages of the exchange of the flows, they are indicating a manifestation of the fermion exchange of the localizing nature.

hep-th↗

arXiv2024

Harmonic Oscillator with a Step and its Isospectral Properties

We investigate the one-dimensional Schrödinger equation for a harmonic oscillator with a finite jump $a$ at the origin. The solution is constructed by employing the ordinary matching-of-wavefunctions technique. For the special choices of $a$, $a=4\ell$ ($\ell=1,2,\ldots$), the wavefunctions can be expressed by the Hermite polynomials. Moreover, we explore isospectral deformations of the potential via the Darboux transformation. In this context, infinitely many isospectral Hamiltonians to the ordinary harmonic oscillator are obtained.

math-ph↗

arXiv2023

Skyrmions and pion stars in the $U(1)$ gauged Einstein-Skyrme model

We consider topological and non-topological regular soliton solutions in the Einstein-Maxwell-Skyrme theory. We analyze the properties of these solutions and determine their domains of existence. The dependence of the solutions on the gauge coupling and on the strength of the effective gravitational coupling are examined. Topologically trivial localized field configurations, \textit{pion stars}, are shown to exist, as non-linear gravitational bound states of the Skyrme field. Both spherically-symmetric and axially-symmetric pion stars are considered. We find that these solutions share many features with the usual (mini-)boson stars. In particular they also exhibit a spiraling behavior and do not possess a flat space limit.

hep-th↗

arXiv2023

A baby--Skyrme model with anisotropic DM interaction: Compact skyrmions revisited

We consider a baby--Skyrme model with Dzyaloshinskii--Moriya interaction (DMI) and two types of potential terms. The model has a close connection with the vacuum functional of fermions coupled with $O(3)$ nonlinear $\bm{n}$-fields and with a constant $SU(2)$ gauge background. The energy functional is derived from the heat-kernel expansion for the fermion determinant. The model possesses normal skyrmions with topological charge $Q = 1$. The restricted version of the model also includes both the weak-compacton case (at the boundary, not continuously differentiable) and genuine-compacton case (continuously differentiable). The model consists of only the Skyrme term, and the DMI provides soliton solutions that are known as \textit{skyrmions without any potential}. The BPS equation in the supersymmetric soliton models implies that the impurity coupling is closely related to the DMI. Therefore, the effect of an exponentially localized DMI is also studied in the present model.

hep-th↗

arXiv2023

SWKB Quantization Condition for Conditionally Exactly Solvable Systems and the Residual Corrections

The SWKB quantization condition is an exact quantization condition for the conventional shape-invariant potentials. On the other hand, this condition equation does not hold for other known solvable systems. The origin of the (non-)exactness is understood in the context of the quantum Hamilton--Jacobi formalism. First, we confirm the statement and show inexplicit properties numerically for the case of the conditionally exactly solvable systems by Junker and Roy. The SWKB condition breaks for this case, but the condition equation is restored within a certain degree of accuracy. We propose a novel approach to evaluate the residual by perturbation, intending to explore the correction terms for the SWKB condition equation.

quant-ph↗

arXiv2022

Mock-integrability and stable solitary vortices

Localized soliton-like solutions to a $(2+1)$-dimensional hydro-dynamical evolution equation are studied numerically. The equation is so-called Williams-Yamagata-Flierl equation, which governs geostrophic fluid in a certain parameter range. Although the equation does not have an integrable structure in the ordinary sense, we find there exist shape-keeping solutions with very long life in a special background flow and an initial condition. The stability of the localization at the fusion process of two soliton-like objects is also investigated. As for the indicator of the long-term stability of localization, we propose a concept of configurational entropy, which has been introduced in analysis for non-topological solitons in field theories.

math-ph↗

arXiv2021

Phase analyses for compact, charged boson stars and shells harboring black holes in the $\mathbb{C}P^N$ nonlinear sigma model

Phase diagrams of the boson stars and shells of the $U(1)$ gauged $\mathbb{C}P^N$ nonlinear sigma model are studied. The solutions of the model exhibit both the ball- and the shell-shaped charge density depending on $N$. There appear four independent regions of the solutions which are essentially caused from the coexistence of electromagnetism and gravity. We examine several phase diagrams of the boson stars and the shells and discuss what and how the regions are emerged. A coupling with gravity allows for harboring of the charged black holes for the $Q$-shell solutions. Some solutions are strongly affected by the presence of the black holes and they allow to be smoothly connected. As a result, the regions are integrated by the harboring black holes.

hep-th↗

arXiv2021

Isolated Skyrmions in the $CP^2$ nonlinear $σ$-model with a Dzyaloshinskii-Moriya type interaction

We study two dimensional soliton solutions in the $CP^2$ nonlinear $σ$-model with a Dzyaloshinskii-Moriya type interaction. First, we derive such a model as a continuous limit of the $SU(3)$ tilted ferromagnetic Heisenberg model on a square lattice. Then, introducing an additional potential term to the derived Hamiltonian, we obtain exact soliton solutions for particular sets of parameters of the model. The vacuum of the exact solution can be interpreted as a spin nematic state. For a wider range of coupling constants, we construct numerical solutions, which possess the same type of asymptotic decay as the exact analytical solution, both decaying into a spin nematic state.

hep-th↗

arXiv2020

Numerical study of the SWKB condition of novel classes of exactly solvable systems

The supersymmetric WKB (SWKB) condition is supposed to be exact for all known exactly solvable quantum mechanical systems with the shape invariance. Recently, it was claimed that the SWKB condition was not exact for the extended radial oscillator, whose eigenfunctions consisted of the the exceptional orthogonal polynomial, even the system possesses the shapeinvariance.In this paper, we examine the SWKB condition for the two novel classes of exactly solvable systems: one has the multi-indexed Laguerre and Jacobi polynomials as the main parts of the eigenfunctions, and the other has the Krein--Adler Hermite, Laguerre and Jacobipolynomials.For all of them, one can always remove the $\hbar$-dependency from the condition, and it is satisfied with a certain degree of accuracy.

math-ph↗

arXiv2020

Compact, charged boson-stars, -shells in the $\mathbb{C}P^N$ gravitating nonlinear sigma model

We study $U(1)$ gauged gravitating compact $Q$-ball, $Q$-shell solutions in a nonlinear sigma model with the target space $\mathbb{C}P^N$. The models with odd integer $N$ and a special potential can be parameterized by $N$-th complex scalar fields and they support compact solutions. Implementing the $U(1)$ gauge field in the model, the behavior of the solutions become complicated than the global model. Especially, they exhibit branch, i.e., two independent solutions with same shooting parameter. The energy of the solutions in the first branch behaves as $E\sim Q^{5/6}$ for small $Q$, where $Q$ stands for the $U(1)$ Noether charge. For the large $Q$, it gradually deviates from the scaling $E\sim Q^{5/6}$ and, for the $Q$-shells it is $E\sim Q^{7/6}$, which forms the second branch. A coupling with gravity allows for harboring of the Schwarzschild black holes for the $Q$-shell solutions, forming the charged boson shells. The space-time then consist of a charged black hole in the interior of the shell, surrounded by a $Q$-shell, and the outside becomes a Reissner-Nordström space-time. These solutions inherit the scaling behavior of the flat space-time.

hep-th↗

arXiv2019

Statistical nature of Skyrme-Faddeev models in $2+1$ dimensions and normalizable fermions

The Skyrme-Faddeev model has planar soliton solutions with target space $\mathbb{C}P^N$. An Abelian Chern-Simons term (the Hopf term) in the Lagrangian of the model plays a crucial role for the statistical properties of the solutions. Because $Π_3(\mathbb{C}P^1)=\mathbb{Z}$, the term becomes an integer for $N=1$. On the other hand, for $N>1$, it becomes perturbative because $Π_3(\mathbb{C}P^N)$ is trivial. The prefactor $Θ$ of the Hopf term is not quantized, and its value depends on the physical system. We study the spectral flow of the normalizable fermions coupled with the baby-Skyrme model ($\mathbb{C}P^N$ Skyrme-Faddeev model). We discuss whether the statistical nature of solitons can be explained using their constituents, i.e., the quarks.

hep-th↗

arXiv2019

Gravitating compact $Q$-ball and $Q$-shell solutions in the $\mathbb{C}P^N$ nonlinear sigma model

We study compact gravitating $Q$-ball, $Q$-shell solutions in a sigma model with the target space $\mathbb{C}P^N$. Models with odd integer $N$ and suitable potential can be parameterized by $N$-th complex scalar fields and they support compact solutions. A coupling with gravity allows for harboring of the Schwarzschild black holes for the $Q$-shell solutions. The energy of the solutions behaves as $E\sim |Q|^{5/6}$, where $Q$ stands for the $U(1)$ Noether charge, for both the gravitating and the black hole solutions.Notable difference from the solutions of the flat space is that upper bound of $|Q|$ appears when the coupling with gravity is stronger. The maximal value of $|Q|$ quickly reduces for larger coupling constant. It may give us a useful hint of how a star forms its shape with a certain finite number of particles.

hep-th↗

arXiv2018

$SU(3)$ Knot Solitons: Hopfions in the $F_2$ Skyrme-Faddeev-Niemi model

We discuss the existence of knot solitons (Hopfions) in a Skryme-Faddeev-Niemi-type model on the target space $SU(3)/U(1)^2$, which can be viewed as an effective theory of both the $SU(3)$ Yang-Mills theory and the $SU(3)$ anti-ferromagnetic Heisenberg model. We derive the knot solitons with two different types of ansatz: the first is a trivial embedding configuration of $SU(2)$ into $SU(3)$, and the second is a non-embedding configuration that can be generated through the Bäcklund transformation. The resulting Euler-Lagrange equations for both ansatz reduce exactly to those of the $CP^1$ Skyrme-Faddeev-Niemi model. We also examine some quantum aspects of the solutions using the collective coordinate zero-mode quantization method.

hep-th↗