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Atsushi Nakamula

Publications and source records attributed to Atsushi Nakamula.

At least 19 recordsLinked to original sources
arXiv2026

Topology of Calorons Re-examined

We reconsider the detailed structure of the topological character of the instantons in pure Yang-Mills theory on $S^1\times\mathbb{R}^3$, known as calorons. We argue that the standard formula for the second Chern number requires modification, as revealed by explicit analytic consideration. For concreteness, we compute the second Chern number of the Harrington-Shepard caloron of the two-dimensional special unitary gauge group ($\mathrm{SU}(2)$) with unit topological charge in several gauges. The general formula is shown to remain valid even when the gauge connection is in a singular gauge. The gauge dependence of the magnetic charge is also discussed.

hep-th↗
arXiv2024

Analytic Approach for Computation of Topological Number of Integrable Vortex on Torus

An analytic method to calculate the vortex number on a torus is constructed, focusing on analytic vortex solutions to the Chern-Simons-Higgs theory, whose governing equation is the so-called Jackiw-Pi equation. The equation is one of the integrable vortex equations and is reduced to Liouville's equation. The requirement of continuity of the Higgs field strongly restricts the characteristics and the fundamental domain of the vortices. Also considered are the decompactification limits of the vortices on a torus, in which "flux loss" phenomena occasionally occur.

hep-th↗
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↗
arXiv2020

Symmetric calorons of higher charges and their large period limits

Periodic instantons, also called calorons, are the BPS solutions to the pure Yang-Mills theories on $\mathbb{R}^3\times S^1$. It is known that the calorons interconnect with the instantons and the BPS monopoles as the ratio of their size to the period of $S^1$ varies. We give, in this paper, the action density configurations of the $SU(2)$ calorons of higher instanton charges with several platonic symmetries through the numerical Nahm transform, after the construction of the analytic Nahm data. The calorons considered are 5-caloron with octahedral symmetry, 7-caloron with icosahedral symmetry, and 4-caloron interconnecting tetrahedral and octahedral symmetries. We also consider the large period, or the instanton, limits of the Nahm data, i.e., the ADHM limits, and observe the similar spatial distributions of the action densities with the calorons.

hep-th↗
arXiv2019

Cosmic strings in compactified gauge theory

A solution of the vortex type is given in a six-dimensional $SU(2)\times U(1)$ pure gauge theory coupled to Einstein gravity in a compactified background geometry. We construct the solution of an effective abelian Higgs model in terms of dimensional reduction. The solution, however, has a peculiarity in its physically relevant quantity, a deficit angle, which is given as a function of the ratio of the gauge couplings of $SU(2)$ and $U(1)$. The size of the extra space (sphere) is shown to vary with the distance from the axis of the "string".

hep-th↗
arXiv2018

Condensation of Yang-Mills field at High Temperature in the Presence of Fermions

The possible condensation of the time-component of Yang-Mills field at finite temperature is discussed in the presence of Dirac fermions. We show that the condensation forms regardless of the number of fundamental and adjoint fermion species coupled to the Yang-Mills field. The effect of finite density of fermions is also investigated and it is shown that the magnitude of the condensation is also independent of the densities.

hep-th↗
arXiv2017

Phase Transition and String Formation in Six-Dimensional Gauge Theory

We consider an SU(2) gauge theory in six-dimensions. We show that there exists non-trivial structure of gauge vacua in compactified background M4xS2. In this circumstances, there can exist a cosmic string, whose solution is recently constructed by the present authors. We consider the stage of the formation of the strings, i.e., the phase transition of the gauge configuration on the sphere in the `hot' early universe. We find that if SU(2)-doublet fermions are the dominant component of the radiation at high temperatures, the phase transition successively occurs after the spontaneous-compactification transition. Otherwise, non-trivial vacuum with `monopole' configuration of the SU(2) gauge field never appears globally and strings must be formed at the same time as the extra space is compactified.

hep-th↗
arXiv2017

ADHM Construction of (Anti-)Self-dual Instantons in Eight Dimensions

We study the ADHM construction of (anti-)self-dual instantons in eight dimensions. We propose the general scheme to construct the (anti-)self-dual gauge field configurations $F \wedge F = \pm *_8 F \wedge F$ whose finite topological charges are given by the fourth Chern number. We show that our construction reproduces the known $\text{SO}(8)$ one-instanton solution. We also construct multi-instanton solutions of 't Hooft and Jackiw-Nohl-Rebbi (JNR) types in the dilute instanton gas approximation. The well-separated configurations of multi-instantons reproduce the correct topological charges with high accuracy. We also show that our construction is generalized to (anti-)self-dual instantons in $4n \ (n=3,4, \ldots)$ dimensions.

hep-th↗
arXiv2017

Atiyah-Manton Construction of Skyrmions in Eight Dimensions

We show that the eight-dimensional instanton solution, which satisfies the self-duality equation $F \wedge F = *_8 F \wedge F$, realizes the static Skyrmion configuration in eight dimensions through the Atiyah-Manton construction. The relevant energy functional of the Skyrme field is obtained by the formalism developed by Sutcliffe. By comparing the Skyrmion olution associated with the extreme of the energy, with the Atiyah-Manton solution constructed by the instantons, we find that they agree with high accuracy. This is a higher-dimensional analogue of the Atiyah-Manton construction of Skyrmions in four dimensions. Our result indicates that the instanton/Skyrmion correspondence seems to be an universal property in $4k \ (k=1, 2, \ldots)$ dimensions.

hep-th↗
arXiv2015

Double Compactification

A cosmological scenario according to which our universe experienced space-time compactifications twice in its early development is investigated through toy models. In this scenario gauge configurations on an extra space play essential roles to bring about a change of the dimensionality of the compactified space. Simple models are offered and their behaviour at finite temperature is examined. A possibility of causing inflation and problems on our scenario is argued briefly.

gr-qc↗
arXiv2015

The Universe as a Topological Defect in a Higher-Dimensional Einstein-Yang-Mills Theory

An interpretation is suggested that a spontaneous compactification of space-time can be regarded as a topological defect in a higher-dimensional Einstein-Yang-Mills (EYM) theory. We start with $D$-dimensional EYM theory in our present analysis. A compactification leads to a $D-2$ dimensional effective action of Abelian gauge-Higgs theory. We find a "vortex" solution in the effective theory. Our universe appears to be confined in a center of a "vortex", which has $D-4$ large dimensions. In this paper we show an example with $SU(2)$ symmetry in the original EYM theory, and the resulting solution is found to be equivalent to the "instanton-induced compactification". The cosmological implication is also mentioned.

gr-qc↗
arXiv2014

Wilson-Loop Symmetry Breaking Reexamined

The splitting in energy of gauge field vacua on the non-simply connected space $S^3/Z_2$ is reconsidered. We show the calculation to the one-loop level for a Yang-Mills vector with a ghost field. We confirm our previous result and give a solution to the question posed by Freire, Romão and Barroso.

hep-th↗
arXiv2014

Gauge Fields on Torus and Partition Function of Strings

In this paper we consider the interrelation between compactified string theories on torus and gauge fields on it. We start from open string theories with background gauge fields and derive partition functions by path integral. Since the effects of background fields and compactification correlate only through string zero modes, we investigate these zero modes. From this point of view, we discuss the Wilson loop mechanism at finite temperature. For the closed string, only a few comments are mentioned.

hep-th↗
arXiv2014

Aspects of $C_3$-symmetric calorons from numerical Nahm transform

Calorons are finite action solutions to the anti-selfdual Yang-Mills equations on $\mathbb{R}^3\times S^1$. They are generally constructed by the so called Nahm construction. We perform the numerical Nahm transform for the Nahm data of 3-calorons with $C_3$-symmetry, which do not have the monopole limits. Dissimilar to the cases of having monopole limits, we can trace the zero-circumference limit of $S^1$.It is found that the action density of the calorons tends to fade away as $S^1$ shrinks.

hep-th↗
arXiv2013

Born-Infeld Monopoles and Instantons

We show that in certain non-Abelian Born-Infeld-Higgs theories there are monopole solutions obeying Bogomol'nyi-like equations in the Prasad-Sommerfield limit. We also discuss the existence of instanton solution in an SU(2) Born-Infeld theory.

hep-th↗
arXiv2012

Cyclic Calorons

The Nahm data of periodic instantons, often called calorons, with spatial $C_N$-symmetries are considered, by applying Sutcliffe's ansatz for the monopoles with $C_N$-symmetries. The bulk data of calorons are shown to enjoy the periodic Toda lattice, and the solutions are given in terms of elliptic theta functions. The case of N=3 calorons are investigated in detail. It is found that the "scale parameters" of these calorons have upper bounds in their values, so that they do not have the large scale, or monopole, limits. The instanton limit of the $C_3$-symmetric caloron is obtained.

hep-th↗
arXiv2012

Sigma Model BPS Lumps on Torus

We study doubly periodic Bogomol'nyi-Prasad-Sommerfield (BPS) lumps in supersymmetric CP^{N-1} non-linear sigma models on a torus T^2. Following the philosophy of the Harrington-Shepard construction of calorons in Yang-Mills theory, we obtain the n-lump solutions on compact spaces by suitably arranging the n-lumps on R^2 at equal intervals. We examine the modular invariance of the solutions and find that there are no modular invariant solutions for n=1,2 in this construction.

hep-th↗