arXiv subjects

Masayuki Asakawa

Publications and source records attributed to Masayuki Asakawa.

At least 19 recordsLinked to original sources

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↗

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.

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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.

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arXiv2021

Single static-quark system above $T_c$ investigated by energy-momentum tensor in SU(3) Yang-Mills theory

We investigate the distribution of energy-momentum tensor (EMT) around a static quark in the deconfined phase of SU(3) Yang-Mills theory. The EMT defined through the gradient-flow formalism is used for the numerical analysis of the EMT distribution around the Polyakov loop with the continuum extrapolation. Using EMT, one can study the mechanical distortion of the color gauge field induced by the static charge. We find substantial separation in the absolute values of the EMT eigenvalues which is not observed in Maxwell theory. The separation grows as temperature is lowered toward the critical temperature. The lattice data also indicate the thermal screening at long distance and the perturbative behavior at short distance.

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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.

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arXiv2020

Distribution of Energy-Momentum Tensor around a Static Quark in the Deconfined Phase of SU(3) Yang-Mills Theory

Energy momentum tensor (EMT) characterizes the response of the vacuum as well as the thermal medium under the color electromagnetic fields. We define the EMT by means of the gradient flow formalism and study its spatial distribution around a static quark in the deconfined phase of SU(3) Yang-Mills theory on the lattice. Although no significant difference can be seen between the EMT distributions in the radial and transverse directions except for the sign, the temporal component is substantially different from the spatial ones near the critical temperature $T_c$. This is in contrast to the prediction of the leading-order thermal perturbation theory. The lattice data of the EMT distribution also indicate the thermal screening at long distance and the perturbative behavior at short distance.

hep-lat↗

arXiv2020

Issues with Search for Critical Point in QCD with Relativistic Heavy Ion Collisions

A systematic search for a critical point in the phase diagram of QCD matter is underway at the Relativistic Heavy Ion Collider (RHIC) and is planned at several future facilities. Its existence, if confirmed, and its location will greatly enhance our understanding of QCD. In this note we emphasize several important issues that are often not fully recognized in theoretical interpretations of experimental results relevant to the critical point search. We discuss ways in which our understanding on these issues can be improved.

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arXiv2020

Comment on "Heavy Quarkonium in Extreme Conditions"

In a recent paper (arXiv:1912.02253), Rothkopf claims that the Bryan method, which is widely used to obtain the solution in the maximum entropy method and makes use of the singular value decomposition of a matrix, limits the search space for the solution. He even presents a counterexample to the Bryan method. In this comment, we first recapitulate the mathematical basis of the Bryan method, and reconfirm that it makes use of no approximations and that it is therefore mathematically rigorous. In the second part, we explicitly show that Rothkopf's ``counterexample'' actually does not constitute a counterexample on the basis of the definition of singular value decomposition itself.

hep-ph↗

arXiv2019

Stress distribution in quark--anti-quark and single quark systems at nonzero temperature

We explore the distribution of the energy momentum tensor (EMT) around quark--anti-quark and single quark at nonzero temperature in SU(3) Yang-Mills gauge theory by extending our previous study on the EMT distribution in static quark--anti-quark systems in vacuum. We discuss the disappearance of the flux tube structure observed in the vacuum simulation. We investigate the total stress acting on the mid-plane between a quark and an anti-quark and show that it agrees with the force obtained from the derivative of the free energy. The color Debye screening effect in the deconfined phase is also discussed in terms of the EMT distribution.

hep-lat↗

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.

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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.

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arXiv2019

Linear confinement and stress-energy tensor around static quark and anti-quark pair -- Lattice simulation with Yang-Mills gradient flow --

We study the spatial distribution of the stress tensor around static quark-anti-quark pair in SU(3) lattice gauge theory. In particular, we reveal the transverse structure of the stress tensor distribution in detail by taking the continuum limit. The Yang-Mills gradient flow plays a crucial role to make the stress tensor well-defined and derivable from the numerical simulations on the lattice.

hep-lat↗

arXiv2019

Distribution of Stress Tensor around Static Quark--Anti-Quark from Yang-Mills Gradient Flow

The spatial distribution of the stress tensor around the quark--anti-quark ($Q\bar{Q}$) pair in SU(3) lattice gauge theory is studied. The Yang-Mills gradient flow plays a crucial role to make the stress tensor well-defined and derivable from the numerical simulations on the lattice. The resultant stress tensor with a decomposition into local principal axes shows, for the first time, the detailed structure of the flux tube along the longitudinal and transverse directions in a gauge invariant manner. The linear confining behavior of the $Q\bar{Q}$ potential at long distances is derived directly from the integral of the local stress tensor.

hep-lat↗

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.

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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.

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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.

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arXiv2017

Correlations of Energy-Momentum Tensor via Gradient Flow in SU(3) Yang-Mills Theory at Finite Temperature

Euclidean two-point correlators of the energy-momentum tensor (EMT) in SU(3) gauge theory on the lattice are studied on the basis of the Yang-Mills gradient flow. The entropy density and the specific heat obtained from the two-point correlators are shown to be in good agreement with those from the one-point functions of EMT. These results constitute a first step toward the first principle simulations of the transport coefficients with the gradient flow.

hep-lat↗

arXiv2017

Photon production spectrum above $T_c$ with a lattice quark propagator

The photon production rate from the deconfined medium is analyzed with the photon self-energy constructed from the quark propagator obtained by the numerical simulation on the quenched lattice for two values of temperature, $T=1.5T_{\rm c}$ and $3T_{\rm c}$, above the critical temperature $T_{\rm c}$. The photon self-energy is calculated by the Schwinger-Dyson equation with the lattice quark propagator and a vertex function determined so as to satisfy the Ward-Takahashi identity. The obtained photon production rate exhibits a similar behavior as the perturbative results at the energy of photons larger than $0.5$~GeV.

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