arXiv subjects

Koichi Murase

Publications and source records attributed to Koichi Murase.

At least 19 recordsLinked to original sources

arXiv2026

Achieving angular-momentum conservation with physics-informed neural networks in computational relativistic spin hydrodynamics

We propose physics-informed neural networks (PINNs) as a numerical solver for relativistic spin hydrodynamics and demonstrate that the total angular momentum, i.e., the sum of orbital and spin angular momentum, is accurately conserved throughout the fluid evolution by imposing the conservation law directly in the loss function as a training target. This enables controlled numerical studies of the mutual conversion between spin and orbital angular momentum, a central feature of relativistic spin hydrodynamics driven by the rotational viscous effect. We present two physical scenarios with a rotating fluid confined in a cylindrical container: one case in which initial orbital angular momentum is converted into spin angular momentum in analogy with the Barnett effect, and the opposite case in which initial spin angular momentum is converted into orbital angular momentum in analogy with the Einstein-de Haas effect. We investigate these conversion processes governed by the rotational viscous effect by analyzing the spacetime profiles of thermal vorticity and spin potential. Our PINNs-based framework provides the first numerical evidence for spin-orbit angular momentum conversion with fully nonlinear computational relativistic spin hydrodynamics.

physics.flu-dyn↗

arXiv2026

Physics-informed neural networks for angular-momentum conservation in computational relativistic spin hydrodynamics

Theoretical developments in relativistic spin hydrodynamics, which describes the macroscopic transport of spin angular momentum alongside other fundamental conserved quantities, have progressed rapidly since the experimental observation of the global spin polarization of $Λ$ hyperons in relativistic heavy-ion collision experiments. However, numerical simulations of relativistic spin hydrodynamics remain largely unaddressed due to computational challenges, particularly the accurate numerical conservation of total angular momentum. In this work, we propose the use of physics-informed neural networks (PINNs) for computational relativistic spin hydrodynamics. As a concrete application, we consider a rotating fluid confined within a cylindrical container. We show that angular-momentum conservation can be accurately achieved in the PINNs-based numerical framework. Furthermore, we investigate the spin-orbit conversion induced by the rotational viscous effect, which is the intrinsic dissipative process of relativistic spin hydrodynamics. Our analysis numerically identifies the mismatch between the transverse thermal vorticity and the spin potential as the driving mechanism of the spin-orbit conversion.

hep-ph↗

arXiv2025

Regularized Lednicky-Lyuboshitz formula for higher partial waves in femtoscopy

Femtoscopy is one of the promising experimental approaches to put constraints on interactions between various species of hadrons from the momentum correlation functions measured in high-energy nuclear collision experiments. The Koonin-Pratt and Lednicky-Lyuboshitz formulae provide useful expressions of the correlation functions and have been widely used to analyze the experimental data based on the assumption that the effect of higher partial waves is negligible. Those formulae can be generalized for higher partial waves, but the generalized Lednicky-Lyuboshitz formula produces wrong results due to a singular behavior of the asymptotic wave function at the origin. In this study, we attempt to solve the problem by regularizing the generalized Lednicky-Lyuboshitz formula with a cutoff and validate it using the Koonin-Pratt formula as a reference. We also show the relationship between the cutoff in the regularized Lednicky-Lyuboshitz and the effective-range correction in the original Lednicky-Lyuboshitz formula. Using the obtained formula, we investigate the source-size dependence, the validity of the effective range expansion, and the cutoff dependence of the correlation function. We also discuss the interaction dependence using the heatmap as a function of the scattering-length parameter $1/a_l$ and the momentum $q$.

nucl-th↗

arXiv2025

$Λ$ and $Σ$ potentials in dense matter based on chiral EFT: Bridging heavy-ion collisions, hypernuclei, and neutron stars

The $Λ$ and $Σ$ directed flows at $\sqrt{s_{NN}}=4.5~\mathrm{GeV}$ are investigated to examine their sensitivity to the hyperon single-particle potentials. The single-particle potentials are obtained from $G$-matrix calculations with two- and three-body forces based on SU(3) chiral effective field theory. The $Λ+Σ^0$ directed flow shows sensitivity to the variation in the $Σ$ single-particle potential. Its effect is more pronounced for the $Σ^0$ directed flow.

nucl-th↗

arXiv2025

Covariant formulation of relativistic quantum molecular dynamics for a system of interacting wave packets

We present a new formulation for the mean-field propagation part of relativistic quantum molecular dynamics, simulating an $N$-body system of interacting Gaussian wave packets via Lorentz scalar and vector potentials. Covariant equations of motion are derived based on the principle of least action with a weak form of mass-shell conditions and time-fixation constraints defined with respect to a chosen foliation. However, as is common with traditional relativistic quantum molecular dynamics, the dynamics exhibits a residual dependence on the chosen foliation, which is unavoidable because a finite number of interacting degrees of freedom is not strictly compatible with relativity. Nevertheless, we show that this dependence remains small for physically reasonable choices of the foliation in practical applications. By introducing a new approximation method to the spatial integral in the equations of motion, these covariant equations of motion can be solved with a computational cost comparable to that of conventional noncovariant quantum molecular dynamics. Furthermore, the new equations of motion accurately estimate the density-dependent potential, as demonstrated through comparison of the forces with the numerical integration. We apply them to $N$-body systems interacting via the Skyrme-type potentials or the relativistic mean field to simulate heavy-ion collisions. Our results show that the derived equations of motion provide a robust approximation to the dynamics of the full numerical integrations.

hep-ph↗

arXiv2025

Dynamical evolution of critical fluctuations with second-order baryon diffusion coupled to chiral condensate

We develop a dynamical model to describe critical fluctuations in heavy-ion collisions, incorporating the baryon diffusion current and chiral condensate as dynamical degrees of freedom, to address their nontrivial scale separation. The model couples fluctuations of the chiral condensate $σ$ with baryon density fluctuations $n$ and the diffusion current $ν$ based on a second-order diffusion equation with a finite relaxation time of the baryon diffusion $τ_\mathrm{R}$. We analyze the spacetime evolution and these correlation functions of the fluctuations in one-dimensionally expanding background. We confirm that an appropriate relaxation time $τ_\mathrm{R}$ ensures causality. We show that propagating waves with finite $τ_\mathrm{R}$ split into two modes at the critical temperature due to a rapid change of kinetic coefficients. In the correlation functions, we find that dynamical $σ$ blurs the structure and peak around the critical temperature. With finite $τ_\mathrm{R}$, the effect of the critical fluctuations persists longer into the later stages of the evolution. These findings suggest importance of dynamical effects of the chiral condensate and baryon diffusion current in identifying critical-point signals in heavy-ion collisions, where the scale separation is nontrivial.

nucl-th↗

arXiv2025

$Λ$ and $Σ$ potentials in neutron stars, hypernuclei, and heavy-ion collisions

With an appropriate $YNN$ force, the $Λ$ single-particle potential ($Λ$ potential) can be made strongly repulsive at high density, and one can solve the hyperon puzzle of neutron stars. We investigate the consistency of such a $Λ$ potential, evaluated recently from $YN$ and $YNN$ forces based on chiral effective field theory, with hypernuclear data and heavy-ion collision data. It is found that model calculations with such a $Λ$ potential can reproduce the data of the $Λ$ hypernuclear spectroscopy and the $Λ$ directed flow in heavy-ion collisions. Also, we evaluate the $Σ$ potential, which can be calculated by using the same hyperon forces as for the $Λ$ potential. Specifically, we show that the low-energy constants characterizing the strength of the $YNN$ force can be chosen to suppress the appearance of the $Λ$'s in neutron stars while at the same time the empirical value of the $Σ$ potential is reproduced.

nucl-th↗

arXiv2024

Higher partial waves in femtoscopy

Femtoscopy is recently gaining more attention as a new approach complementary to scattering experiments for constraining hadron-hadron interactions. We discuss the effect of higher partial waves on the two-particle correlation function, which has been neglected in traditional formulae used in the femtoscopy analyses. We consider the partial-wave expansion of the wave function in the Koonin-Pratt formula to give the correction to the correlation function by a sum of the contributions from each partial wave. We also generalize the Lednicky-Lyuboshitz formula, which was originally derived for the s-wave interaction, for higher partial waves. We find a compact representation of the generalized Lednicky-Lyuboshitz formula given by the backward scattering amplitude $f(θ= π)$ and the Fourier-Laplace transform of the relative source function, which gives an insight into the structure of the Lednicky-Lyuboshitz formula and its relation to the optical theorem. We numerically demonstrate the significance of higher partial waves with resonances using the square potential well. Also, the generalized Lednicky-Lyuboshitz formula turned out to be broken for higher partial waves, which suggests the importance of the centrifugal force for the higher partial waves.

nucl-th↗

arXiv2024

Efficient solver of relativistic hydrodynamics with implicit Runge-Kutta method

We propose a new method to solve the relativistic hydrodynamic equations based on implicit Runge-Kutta methods with a locally optimized fixed-point iterative solver. For numerical demonstration, we implement our idea for ideal hydrodynamics using the one-stage Gauss-Legendre method as an implicit method. The accuracy and computational cost of our new method are compared with those of explicit ones for the (1+1)-dimensional Riemann problem, as well as the (2+1)-dimensional Gubser flow and event-by-event initial conditions for heavy-ion collisions generated by TrENTo. We demonstrate that the solver converges with only one iteration in most cases, and as a result, the implicit method requires a smaller computational cost than the explicit one at the same accuracy in these cases, while it may not converge with an unrealistically large $Δt$. By showing a relationship between the one-stage Gauss-Legendre method with the iterative solver and the two-step Adams-Bashforth method, we argue that our method benefits from both the stability of the former and the efficiency of the latter.

nucl-th↗

arXiv2024

Uncertainty quantification in the machine-learning inference from neutron star probability distribution to the equation of state

We discuss the machine-learning inference and uncertainty quantification for the equation of state (EoS) of the neutron star (NS) matter directly using the NS probability distribution from the observations. We previously proposed a prescription for uncertainty quantification based on ensemble learning by evaluating output variance from independently trained models. We adopt a different principle for uncertainty quantification to confirm the reliability of our previous results. To this end, we carry out the MC sampling of data to infer an EoS and take the convolution with the probability distribution of the observational data. In this newly proposed method, we can deal with arbitrary probability distribution not relying on the Gaussian approximation. We incorporate observational data from the recent multimessenger sources including precise mass measurements and radius measurements. We also quantify the importance of data augmentation and the effects of prior dependence.

nucl-th↗

arXiv2023

Repulsive $Λ$ potentials in dense neutron star matter and binding energy of $Λ$ in hypernuclei

The repulsive three-body force between the lambda ($Λ$) hyperon and medium nucleons is a key element in solving the hyperon puzzle in neutron stars. We investigate the binding energies of $Λ$ hyperon in hypernuclei to verify the repulsive $Λ$ potentials from the chiral effective field theory ($χ$EFT) employing the Skyrme Hartree-Fock method. We find that the $χ$EFT $Λ$ potential with the $ΛNN$ three-body forces reproduces the existing hypernuclear binding energy data, whereas the $Λ$ binding energies are overestimated without the $ΛNN$ three-body force. Additionally, we search for the parameter space of the $Λ$ potentials by varying the Taylor coefficients of the $Λ$ potential and the effective mass of $Λ$ at the saturation density. Our analysis demonstrates that the parameter region consistent with the $Λ$ binding energy data spans a wide range of the parameter space, including even more repulsive potentials than the $χ$EFT prediction. We confirm that these strong repulsive $Λ$ potentials suppress the presence of $Λ$ in the neutron star matter. We found that the $Λ$ potentials repulsive at high densities are favored when the depth of the $Λ$ potential at the saturation density, $U_Λ(ρ_0)=J_Λ$, is $J_Λ\gtrsim-29~\text{MeV}$, while attractive ones are favored when $J_Λ\lesssim -31~\text{MeV}$. This suggests that the future high-resolution data of hypernuclei could rule out the scenario in which $Λ$s appear through the precise determination of $J_Λ$ within the accuracy of $1~\text{MeV}$.

nucl-th↗

arXiv2023

A Poincaré covariant cascade method for high-energy nuclear collisions

We present a Poincaré covariant cascade algorithm based on the constrained Hamiltonian dynamics in an $8N$-dimensional phase space to simulate the Boltzmann-type two-body collision term. We compare this covariant cascade algorithm with traditional $6N$-dimensional phase-space cascade algorithms. To validate the covariant cascade algorithm, we perform box calculations. We examine the frame dependence of the algorithm in a one-dimensionally expanding system as well as the compression stages of colliding two nuclei. We confirm that our covariant cascade method is reliable to simulate high-energy nuclear collisions. Furthermore, we present Lorentz-covariant equations of motion for the $N$-body system interacting via potentials, which can be efficiently solved numerically.

nucl-th↗

arXiv2023

Hydrodynamic fluctuations and ultra-central flow puzzle in heavy-ion collisions

One of the long-standing problems in the field of high-energy heavy-ion collisions is that the dynamical models based on viscous hydrodynamics fail to describe the experimental elliptic flow $v_2$ and the triangular flow $v_3$ simultaneously in ultra-central collisions. The problem, known as the "ultra-central flow puzzle", is specifically that hydrodynamics-based models predict the flow ratio of the two-particle cumulant method $v_2\{2\}/v_3\{2\} > 1$ while $v_2\{2\}/v_3\{2\} \sim 1$ in the experimental data. In this Letter, we focus on the effects of hydrodynamic fluctuations during the space-time evolution of the QGP fluid on the flow observables in the ultra-central collisions. Using the (3+1)-dimensional integrated dynamical model which includes relativistic fluctuating hydrodynamics, we analyze the anisotropic flow coefficients $v_n\{2\}$ in 0-0.2% central Pb+Pb collisions at $\sqrt{s_\text{NN}}=2.76~\text{TeV}$. We find that the hydrodynamic fluctuations decrease the model overestimate of $v_2\{2\}/v_3\{2\}$ from the experimental data by about 19% within the present setup of $η/s = 1/2π$. This means that the hydrodynamic fluctuations qualitatively have an effect to improve the situation for the puzzle, but the effect of the hydrodynamic fluctuations alone is quantitatively insufficient to resolve the puzzle. The decrease of the ratio largely depends on the shear viscosity $η/s$, which calls for future comprehensive analyses with, for example, a realistic temperature-dependent viscosity.

nucl-th↗

arXiv2022

Directed flow of $Λ$ from heavy-ion collisions and hyperon puzzle of neutron stars

We examine the $Λ$ potential from the chiral effective field theory ($χ$EFT) via the $Λ$ directed flow from heavy-ion collisions. We implement the $Λ$ potential obtained from the $χ$EFT in a vector potential version of relativistic quantum molecular dynamics. We find that the $Λ$ potentials obtained from the $χ$EFT assuming weak momentum dependence reproduce the $Λ$ directed flow measured by the STAR collaboration in the Beam Energy Scan program. While the $Λ$ directed flow is not very sensitive to the density dependence of the potential, the directed flow at large rapidities is susceptible to the momentum dependence. Thus understanding the directed flow of hyperons in a wide range of beam energy and rapidity is helpful in understanding hyperon potentials in dense matter.

nucl-th↗

arXiv2022

Directed flow of $Λ$ in high-energy heavy-ion collisions and $Λ$ potential in dense nuclear matter

We investigate the sensitivity of the $Λ$ directed flow to the $Λ$ potential in mid-central Au + Au collisions at $\sqrt{s_{NN}}\approx3.0$--$30$ GeV. The $Λ$ potential obtained from the chiral effective field theory ($χ$EFT) is used in a microscopic transport model, a vector version of relativistic quantum molecular dynamics (RQMDv). We find that the density-dependent $Λ$ potentials, obtained from the $χ$EFT assuming weak momentum dependence of the potential, reproduce the rapidity and the beam-energy dependence of the $Λ$ directed flow measured by the STAR collaboration in the Beam Energy Scan program. Although the $Λ$ directed flow is insensitive to the density dependence of the potential, it is susceptible to the momentum dependence. We also show that a hydrodynamics picture based on the blast-wave model predicts a similarity of the proton, $Λ$, and $Ξ$ directed flows, but the directed flow of $Ω$ baryons slightly deviates from other baryons. We also show that the quark coalescence predicts different rapidity dependence of the directed flows for hyperons. These investigations suggest that measurements of a wide range of the rapidity dependence of the directed flow of hyperons may provide important information about the properties of hot and dense matter created in high-energy heavy-ion collisions.

nucl-th↗

arXiv2022

Scales in light-nuclei production near the QCD critical point

Based on the coalescence model, we analyse the light-nuclei production near the critical point by expanding the phase-space distribution function $f(\mathbf{r},\mathbf{p})$ in terms of the phase-space cumulants $\sim \langle r^m p^m\rangle_c$. We show that the dominant contribution of the phase-space distribution to the yield of light nuclei is determined by the second-order phase-space cumulants. Here, we identify the fireball size, the homogeneity length, and the effective temperature, which are encoded in the second-order phase-space cumulants, as the relevant scales in explaining the yield of light nuclei. These scales are typically much larger than the correlation length of the critical fluctuations created in the rapid expansion of the heavy-ion systems, so we need to eliminate this dominant contribution of the relevant scales in order to isolate the critical contribution from the yield of light nuclei. We find that the second-order phase-space cumulants appeared in the yields of light-nuclei with different mass numbers share a similar structure. This property allows us to construct ratios of light-nuclei yields in appropriate combinations so that the effect of the relevant scales of the light-nuclei yield cancels, which isolates the critical effects.

nucl-th↗

arXiv2022

Examination of background effects on light-nuclei yield ratio in relativistic heavy-ion collisions

The light-nuclei yield ratio is one of the candidates to probe the critical fluctuations of hot QCD matter. In this paper, we investigate the \textit{background effects}, namely the non-critical effects coming from the non-trivial thermal background, on the light-nuclei production within the framework of the coalescence model. Specifically, we analyze the impact of the equilibrium phase-space distribution function of nucleons, $f(\mathbf{r},\mathbf{p})$, on the light-nuclei yield ratio $N_tN_p/N_d^2$, where $N_t$, $N_p$, and $N_d$ denote triton, proton, and deuteron yields. By considering the characteristic function of the phase-space distribution, we systematically expand the yield of light nuclei of $A$-constituent nucleons, $N_A$, in terms of the \textit{phase-space cumulants}, $\langle\mathbf{r}^n\mathbf{p}^m\rangle_c$. We find that the cumulants up to the second-order are canceled out in the generalized ratio $N_p^{B-A} N_B^{A-1}/N_A^{B-1}$. This means that the dominant background effects including the fireball size, the kinetic freeze-out temperature, and the coordinate--momentum correlations caused by the radial expansion play an insignificant role in the yield ratio, which supports the yield ratio as a useful tool for the critical-point search. We also show several examples of background phase-space distributions for the qualitative illustration. The higher-order cumulants, which correspond to the non-Gaussian shape of the phase-space profile, play an important role in the variation of the yield ratio particularly for the smaller fireball sizes. Qualitatively, the spatial structure of the background decreases the yield ratio, and the azimuthal anisotropy $v_n$ increases it. The higher order of the azimuthal anisotropy causes a larger effect on the yield ratio. These results call for the comprehensive future studies of the yield ratio using sophisticated dynamical models.

nucl-th↗

arXiv2021

Effects of hydrodynamic and initial longitudinal fluctuations on rapidity decorrelation of collective flow

We investigate the interplay between hydrodynamic fluctuations and initial longitudinal fluctuations for their effects on the rapidity decorrelation of collective flow in high-energy nuclear collisions. We use a (3+1)-dimensional integrated dynamical model in which we combine initial conditions with longitudinal fluctuations, fluctuating hydrodynamics and hadronic cascades. We analyse the factorisation ratio in the longitudinal direction to study the effect of these fluctuations on the rapidity decorrelation. We find an essential difference between the effects of the hydrodynamic fluctuations and the initial longitudinal fluctuations in the centrality dependence of the factorisation ratios. A combination of the hydrodynamic fluctuations and the initial longitudinal fluctuations leads to reproduction of the centrality dependence of the second-order factorisation ratio, $r_2(η_\mathrm{p}^\mathrm{a},η_\mathrm{p}^\mathrm{b})$, measured by the CMS Collaboration. Our model also qualitatively describes the centrality dependence of the third-order factorisation ratio, $r_3(η_\mathrm{p}^\mathrm{a},η_\mathrm{p}^\mathrm{b})$. These results demonstrate the importance of the hydrodynamic fluctuations, as well as the initial longitudinal fluctuations, in understanding the longitudinal dynamics of high-energy nuclear collision reactions.

nucl-th↗