arXiv subjects

Yukinao Akamatsu

Publications and source records attributed to Yukinao Akamatsu.

At least 19 recordsLinked to original sources

arXiv2026

Effective field theory for dissipative photons from higher-form symmetries

Recent developments in generalized symmetries have provided new insights into quantum field theories. Within this framework, photons can be understood as Nambu-Goldstone modes associated with a spontaneously broken higher-form symmetry. In this work, we develop an effective field theory that builds on this symmetry structure to describe the real-time dynamics of photons in insulating media at finite temperature. Combining the Schwinger-Keldysh formalism with the generalized coset construction, we formulate a symmetry-based effective action that incorporates both conservative and dissipative effects. The effective theory implements the dynamical Kubo-Martin-Schwinger symmetry, ensuring consistency with the fluctuation-dissipation relation and Onsager's reciprocal relations. Within this framework, we derive the entropy current associated with dissipative photon dynamics and demonstrate the non-negativity of its divergence, in accordance with the second law of thermodynamics. We also clarify the symmetry origin of the gauge redundancy in the unbroken phase within the Schwinger-Keldysh framework, relating it to strong and weak realizations of higher-form symmetries. Our results provide a model-independent effective description of photon dynamics in insulating media at finite temperature.

hep-th↗

arXiv2026

Enhancement of photon emission rate near QCD critical point

We compute photon emission rate enhancement near the QCD critical point using an effective theory of dynamic critical phenomena and derive a universal photon spectrum. The emission rate scales similarly to conductivity, increasing with the correlation length ($ξ$), diverging at the critical point. The spectrum exhibits $ωdN_γ/d^3k \propto ω^{-1/2}$ in the scaling regime, with the transition occurring at a frequency comparable to shear damping rate $ω\sim γ_η/ξ^2$, reflecting the nonequilibrium properties of the near-critical liquid.

hep-ph↗

arXiv2026

Complex-valued in-medium potential between heavy impurities in ultracold atoms

We formulate the induced potential in a finite temperature cold atomic medium between two heavy impurities, or polarons, which is shown to be \textit{complex-valued} in general. The imaginary part of the complex-valued potential describes a decoherence effect, and thus, the resulting Schrödinger equation for the two polarons acquires a non-Hermitian term. We apply the developed formulation to two representative cases of polarons interacting with medium particles through the $s$-wave contact interaction: (i) the normal phase of single-component (i.e., spin-polarized) fermions using the fermionic field theory, and (ii) a superfluid phase using the superfluid effective field theory, which is valid either for a Bose-Einstein condensate (BEC) of a single-component Bose gas or for the BEC-BCS crossover in two-component fermions at a low-energy regime. Computing the leading-order term, the imaginary part of the potential in both cases is found to show a universal $r^{-2}$ behavior at long distance. We propose three experimental ways to observe the effects of the universal imaginary potential in cold atoms.

cond-mat.quant-gas↗

arXiv2025

Complex potential and open system applications in heavy-ions and cold atoms

Since the discovery of the complex potential of quarkonium at high temperatures, quarkonium has been regarded as an open quantum system in the quark-gluon plasma. Recently, a similar issue regarding in-medium bound states of impurities has also emerged in particle physics and cold atomic physics. We will provide an overview of recent advancements in understanding key quantities such as complex potential and transport coefficients for heavy impurities in finite temperature QCD and cold atomic systems.

hep-ph↗

arXiv2024

Drag force near the QCD critical point

We discuss how heavy quark dynamics is affected by the critical fluctuations near the QCD critical point at finite temperature and density. We find that the heavy quark momentum diffusion constant scales as $κ\proptoξ^{z-3-η}$. In the model H scenario, which is widely accepted for the critical dynamics, the exponents are known as $z\simeq 3, η\sim 0.04$ and the critical singularity of $κ$ is not significant if any. In the model B scenario, $z\simeq 4$ and $κ\propto ξ$ is singular near the critical point.

nucl-th↗

arXiv2022

Simulation of Lindblad equations for quarkonium in the quark-gluon plasma

We study the properties of the Lindbladian quantum mechanical evolution of quarkonia with non-Abelian charges (color-singlet and octet) in the quark-gluon plasma. We confirm that heavy quark recoils in the Lindblad equation correctly thermalize quarkonium colorful states within statistical errors from the simulation method. We also demonstrate that the Lindblad equation in the dipole limit can provide an efficient alternative method, which is applicable to a finite time evolution before thermalization and dramatically reduces the numerical cost. Our findings will serve as a foundation for large-scale simulation of quarkonium dynamics in the relativistic heavy-ion collisions.

nucl-th↗

arXiv2021

Quarkonium in Quark-Gluon Plasma: Open Quantum System Approaches Re-examined

Dissociation of quarkonium in quark-gluon plasma (QGP) is a long standing topic in relativistic heavy-ion collisions because it has been believed to signal one of the fundamental natures of the QGP -- Debye screening due to the liberation of color degrees of freedom. Among recent new theoretical developments is the application of open quantum system framework to quarkonium in the QGP. Open system approach enables us to describe how dynamical as well as static properties of QGP influences the time evolution of quarkonium in a coherent way. Currently, there are several master equations for quarkonium corresponding to various scale assumptions, each derived in different theoretical frameworks. In this review, all of the existing master equations are systematically rederived as Lindblad equations in a unified framework. Also, as one of the most relevant descriptions in relativistic heavy-ion collisions, quantum Brownian motion of heavy quark pair in the QGP is studied in detail. The quantum Brownian motion is parametrized by a few fundamental quantities of QGP such as real and imaginary parts of heavy quark potential (complex potential), heavy quark momentum diffusion constant, and thermal dipole self-energy constant, which constitute in-medium self-energy of a static quarkonium. This indicates that the yields of quarkonia such as $J/ψ$ and $Υ$ in the relativistic heavy-ion collisions have the potential to determine these fundamental quantities.

nucl-th↗

arXiv2021

Nonequilibrium evolution of quarkonium in medium

We review recent progress in open quantum system approach to the description of quarkonium in the quark-gluon plasma. A particular emphasis is put on the Lindblad equations for quarkonium and its numerical simulations.

hep-ph↗

arXiv2021

Dynamics of in-medium quarkonia in SU(3) and SU(2) gauge theories

Decoherence dynamics of quarkonia is studied in the high-temperature deconfined phase of SU($N_c$) gauge theories. In particular, we analyze the symmetry properties of SU($N_c$) stochastic potential model and find a novel "event-by-event" symmetry for $N_c=2$ case, similar to the $G$-parity of hadronic systems. This novel symmetry constrains the relation between diagonal and off-diagonal components of quarkonium density matrix, leaving the latter to be finite at late times. We also present one-dimensional numerical simulation of the model, which indicates the usefulness of the complex potential simulations for the quarkonium survival probabilities in relativistic heavy-ion collisions, provided that the effect of dissipation can be neglected.

nucl-th↗

arXiv2020

Trace preserving quantum dynamics using a novel reparametrization-neutral summation-by-parts difference operator

We develop a novel numerical scheme for the simulation of dissipative quantum dynamics following from two-body Lindblad master equations. All defining continuum properties of the Lindblad dynamics, hermiticity, positivity and in particular trace conservation of the evolved density matrix are preserved. The central ingredient is a new spatial difference operator, which not only fulfils the summation by parts (SBP) property but also implements a continuum reparametrization property. Using the time evolution of a heavy-quark anti-quark bound state in a hot thermal medium as an explicit example, we show how the reparametrization neutral summation-by-parts (RN-SBP) operator preserves the continuum properties of the theory.

physics.comp-ph↗

arXiv2020

Approach to thermalization and hydrodynamics

I review recent progress in thermalization in heavy-ion collisions, with particular emphasis on hydrodynamic attractor, and also report recent progress in hydrodynamic fluctuations.

nucl-th↗

arXiv2019

Quantum Brownian motion of a heavy quark pair in the quark-gluon plasma

In this paper we study the real-time evolution of heavy quarkonium in the quark-gluon plasma (QGP) on the basis of the open quantum systems approach. In particular, we shed light on how quantum dissipation affects the dynamics of the relative motion of the quarkonium state over time. To this end we present a novel non-equilibrium master equation for the relative motion of quarkonium in a medium, starting from Lindblad operators derived systematically from quantum field theory. In order to implement the corresponding dynamics, we deploy the well established quantum state diffusion method. In turn we reveal how the full quantum evolution can be cast in the form of a stochastic non-linear Schrödinger equation. This for the first time provides a direct link from quantum chromodynamics (QCD) to phenomenological models based on non-linear Schrödinger equations. Proof of principle simulations in one-dimension show that dissipative effects indeed allow the relative motion of the constituent quarks in a quarkonium at rest to thermalize. Dissipation turns out to be relevant already at early times well within the QGP lifetime in relativistic heavy ion collisions.

nucl-th↗

arXiv2019

Dynamically Integrated Transport Approach for High-Energy Nuclear Collisions at High Baryon Density

To explore the structure of the QCD phase diagram in high baryon density domain, several high-energy nuclear collision experiments in a wide range of beam energies are currently performed or planned using many accelerator facilities. In these experiments search for a first-order phase transition and the QCD critical point is one of the most important topics. To find the signature of the phase transition, experimental data should be compared to appropriate dynamical models which quantitatively describe the process of the collisions. In this study we develop a new dynamical model on the basis of the non-equilibrium hadronic transport model JAM and 3+1D hydrodynamics. We show that the new model reproduce well the experimental beam-energy dependence of hadron yields and particle ratio by the partial thermalization of the system in our core-corona approach.

nucl-th↗

arXiv2018

Transits of the QCD Critical Point

We analyze the evolution of hydrodynamic fluctuations in a heavy ion collision as the system passes close to the QCD critical point. We introduce two small dimensionless parameters $λ$ and $Δ_s$ to characterize the evolution. $λ$ compares the microscopic relaxation time (away from the critical point) to the expansion rate $λ\equiv τ_0/τ_Q$, and $Δ_s$ compares the baryon to entropy ratio, $n/s$, to its critical value, $Δ_s\equiv (n/s - n_c/s_c)/(n_c/s_c)$. We determine how the evolution of critical hydrodynamic fluctuations depends parametrically on $λ$ and $Δ_s$. Finally, we use this parametric reasoning to estimate the critical fluctuations and correlation length for a heavy ion collision, and to give guidance to the experimental search for the QCD critical point.

nucl-th↗

arXiv2018

Bulk viscosity from hydrodynamic fluctuations with relativistic hydro-kinetic theory

Hydro-kinetic theory of thermal fluctuations is applied to a non-conformal relativistic fluid. Solving the hydro-kinetic equations for an isotropically expanding background we find that hydrodynamic fluctuations give ultraviolet divergent contributions to the energy-momentum tensor. After shifting the temperature to account for the energy of non-equilibrium modes, the remaining divergences are renormalized into local parameters, e.g. pressure and bulk viscosity. We also confirm that the renormalization of the pressure and bulk viscosity is universal by computing them for a Bjorken expansion. The fluctuation-induced bulk viscosity reflects the non-conformal nature of the equation of state and is modestly enhanced near the QCD deconfinement temperature.

nucl-th↗

arXiv2018

Dynamically integrated transport approach for heavy-ion collisions at high baryon density

We develop a new dynamical model for high energy heavy-ion collisions in the beam energy region of the highest net-baryon densities on the basis of non-equilibrium microscopic transport model JAM and macroscopic 3+1D hydrodynamics by utilizing a dynamical initialization method. In this model,dynamical fluidization of a system is controlled by the source terms of the hydrodynamic fields. In addition, time dependent core-corona separation of hot regions is implemented. We show that our new model describes multiplicities and mean transverse mass in heavy-ion collisions within a beam energy region of $3<\sqrt{s_{NN}}<30$ GeV. Good agreement of the beam energy dependence of the $K^+/π^+$ ratio is obtained, which is explained by the fact that a part of the system is not thermalized in our core-corona approach.

nucl-th↗

arXiv2018

Quantum dissipation of a heavy quark from a nonlinear stochastic Schrödinger equation

We study the open system dynamics of a heavy quark in the quark-gluon plasma using a Lindblad master equation. Applying the quantum state diffusion approach by Gisin and Percival, we derive and numerically solve a nonlinear stochastic Schrödinger equation for wave functions, which is equivalent to the Lindblad master equation for the density matrix. From our numerical analysis in one spatial dimension, it is shown that the density matrix relaxes to the Boltzmann distribution in various setups (with and without external potentials), independently of the initial conditions. We also confirm that quantum dissipation plays an essential role not only in the long-time behavior of the heavy quark but also at early times if the heavy quark initial state is localized and quantum decoherence is ineffective.

nucl-th↗

arXiv2018

Dynamical dissociation of quarkonia by wave function decoherence

We investigate the real-time evolution of quarkonium bound states in a quark-gluon plasma in one dimension using an improved QCD based stochastic potential model. This model describes the quarkonium dynamics in terms of a Schrödinger equation with an in-medium potential and two noise terms encoding the residual interactions between the heavy quarks and the medium. The probabilities of bound states in a static medium and in a boost-invariantly expanding quark-gluon plasma are discussed. We draw two conclusions from our results: One is that the outcome of the stochastic potential model is qualitatively consistent with the experimental data in relativistic heavy-ion collisions. The other is that the noise plays an important role in order to describe quarkonium dynamics in medium, in particular it causes decoherence of the quarkonium wave function. The effectiveness of decoherence is controlled by a new length scale $l_{\rm corr}$. It represents the noise correlation length and its effect has not been included in existing phenomenological studies.

nucl-th↗