arXiv subjects

Yasushi Nara

Publications and source records attributed to Yasushi Nara.

At least 19 recordsLinked to original sources

arXiv2026

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.

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arXiv2026

Machine learning the impact parameter in heavy-ion collisions at $\sqrt{s_{\rm NN}}$ = 4 and 11 GeV: a cross-check study with UrQMD, AMPT, and JAM

By generating heavy-ion collision data with the ultrarelativistic quantum molecular dynamics (UrQMD) model, a multiphase transport (AMPT) model, and the JAM model, the impact parameter ($b$) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 4 and 11 GeV is reconstructed using supervised learning and unsupervised learning in machine learning (ML). In supervised learning, the performance of ML algorithm is cross-checked by using data obtained from these three transport models. It is found that the typical mean absolute error (MAE) which measures the average magnitude of the absolute difference between the true and predicted $b$ is between 0.2-0.4 fm, even when training ML algorithm with data generated from one model but testing with data from others. While the conventional method (i.e., a polynomial fit to multiplicity as a function of $b$) only works for data generated from the same model. In the classification task, the present ML-based method also shows significantly superior results compared to the traditional approach. In unsupervised learning, the K-means clustering algorithm is used to partition collision events directly from experimental-style observables, showing that the algorithm autonomously identifies six clusters corresponding to different centrality classes without relying on predefined model-based binning. Our study demonstrates the strong robustness of using an ML algorithm trained on transport-model data for impact-parameter determination, and indicates that this method has the potential to be generalized to handle real experimental data.

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arXiv2026

Forward hadron production in pp collisions at LHC energies from an event generator based on the color glass condensate framework

We investigate inclusive forward single-hadron production in high-energy proton--proton collisions using a CGC-inspired Monte Carlo event generator, MC-CGC. We carried out a systematic study of the sensitivity of the running-coupling Balitsky-Kovchegov (rcBK) evolution equation to its initial conditions by comparing three parameterizations: the McLerran-Venugopalan (MV) model and its two HERA DIS-constrained variants, MV$^γ$ and MV$^e$. Our results indicate that the current LHCb data favor the MV$^γ$ and MV$^e$ models, while the differences from the original MV model become more pronounced at higher transverse momentum and at mid-rapidity. As a complementary analysis, we also compared the dilute-dense (DHJ factorization) and dense-dense ($k_T$ factorization) frameworks. We found that the $k_T$ factorization framework provides a better description of the particle production spectra at mid-rapidity than the DHJ framework, where both the projectile and target are in the dense regime at LHC energies. Predictions for the FoCal measurements at ALICE, including the production of identified neutral mesons and jets, are also presented.

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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.

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arXiv2025

Optimal collision-energy range for realizing macroscopic high-baryon-density matter

We investigate the volume and lifetime of the high baryon-density matter created in heavy-ion collisions and estimate the optimal collision-energy range to realize the high baryon-density region over a large spacetime volume. We simulate central collisions of gold ions for the center-of-mass energy per nucleon pair $\sqrt{s_{NN}}=2.4 - 19.6\;{\rm GeV}$ with a microscopic transport model JAM. We discover that the optimal range is around $\sqrt{s_{NN}}=3 - 5\;{\rm GeV}$, where a baryon density exceeding three times the normal nuclear density is realized with a substantially large spacetime volume. Higher and lower energies are disfavored due to short lifetime and low density, respectively. We also point out that event-by-event fluctuations of the spacetime density profile are large, indicating the importance of the event selection in the experimental analysis.

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arXiv2025

Simultaneous description of high density QCD matter in heavy ion collisions and neutron star observations

A combined constraint on the QCD equation of state, at high densities, from connecting neutron star observations to data from heavy ion reactions is presented. We use the Chiral Mean Field Model which can describe neutron star and iso-spin symmetric matter and allows the consistent calculation of the density and momentum dependent potentials of baryons which are then implemented in the UrQMD transport model. In contrast to previous studies, the same equation of state constrained from neutron star properties is also able to describe experimental observables in heavy ion reactions at the HADES experiment. Unlike many other approaches our results are not constraint to densities up to nuclear saturation or perturbative results which allows a continuous description of the equation of state over a large range in baryon density.

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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.

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arXiv2024

Momentum dependent potentials from a parity doubling CMF model in UrQMD: Results on flow and particle production

The quantum molecular dynamics (QMD) part of the UrQMD model is extended to allow implementation of momentum dependent potentials from a parity doubling chiral mean field (CMF) model. Important aspects like energy conservation and effects on particle production and flow are discussed. It is shown, that this new implementation reproduces qualitatively and quantitatively available data over a wide range of beam energies and improves the description of observables without exception. In particular the description of hyperon and pion production at SIS18 energies is improved. From a comparison with HADES data one could conclude that the present parametrization of the CMF model leads to a slightly too weak momentum dependence. However, a more firm conclusion will require a systematic comparison with flow and multiplicity data over a range of beam energies and system sizes. Our work serves as an important step towards such future studies where the properties of dense QCD matter, through parameters of the CMF model, can be constraint using a comparison of the UrQMD model with high precision heavy ion data, finally also allowing direct comparisons with neutron star and neutron star merger observables.

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arXiv2024

Comparing pion production in transport simulations of heavy-ion collisions at $270A$ MeV under controlled conditions

Within the TMEP, we present a detailed study of the performance of different transport models in Sn+Sn collisions at $270A$ MeV, and put particular emphasis on the production of pions and $Δ$ resonances, which have been used as probes of the nuclear symmetry energy. We prescribe a common and rather simple physics model, and follow in detail the results of 4 BUU models and 6 QMD models. The nucleonic evolution of the collision and the nucleonic observables in these codes do not completely converge, but the differences among the codes can be understood as being due to several reasons: the basic differences between BUU and QMD models in the representation of the phase-space distributions, computational differences in the mean-field evaluation, and differences in the adopted strategies for the Pauli blocking in the collision integrals. For pionic observables, we find that a higher maximum density leads to an enhanced pion yield and a reduced $π^-/π^+$ yield ratio, while a more effective Pauli blocking generally leads to a slightly suppressed pion yield and an enhanced $π^-/π^+$ yield ratio. We specifically investigate the effect of the Coulomb force, and find that it increases the total $π^-/π^+$ yield ratio but reduces the ratio at high pion energies, although differences in its implementations do not have a dominating role in the differences among the codes. Taking into account only the results of codes that strictly follow the homework specifications, we find a convergence of the codes in the final charged pion yield ratio to a $1σ$ deviation of about $5\%$. However, the uncertainty is expected to be reduced to about $1.6\%$ if the same or similar strategies and ingredients, i.e., an improved Pauli blocking and calculation of the non-linear term in the mean-field potential, are similarly used in all codes.

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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 $1~\text{MeV}$.

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arXiv2023

Enhanced Dilepton production near the color superconducting phase and the QCD critical point

The dilepton production yields in relativistic heavy ion collisions are investigated along isentropic trajectories in the quark (Wigner) phase within the two-flavor Nambu-Jona-Lasinio model. An enhancement of the ultra-low energy dilepton yield in the vicinity of the color superconducting (CSC) phase and the QCD critical point (QCD-CP) is found, compared to the free quark gas. Furthermore, we have found a nontrivial structure in the beam energy dependence of the ultra-low energy dilepton yield. A local maximum and minimum of the dilepton yield as a function of entropy per baryon emerge when the trajectories are close to both locations of the CSC phase transition line and the QCD-CP. Only the maximum appears in the scenario without the CSC phase but with the QCD-CP. On the other hand, when only the CSC phase is considered, the dilepton yield monotonically increases as the beam energy decreases. These distinctive patterns could potentially serve as signals of the CSC phase and QCD-CP. In addition, it is found that the dilepton production yield and the location of the minimum strongly depend on the value of diquark coupling, suggesting the possibility that the value of the diquark coupling may be extracted from experimental data.

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arXiv2023

Study of bulk properties of the system formed in U+U collisions at $\sqrt{s_{\mathrm NN}}$ =~2.12~GeV using JAM model

The Lanzhou Cooling-Storage-Ring facility is set to conduct experiments involving Uranium-Uranium collisions at the center of mass energies ranging from 2.12 to 2.4 GeV. Our investigation is focused on various bulk observables, which include charged particle multiplicity ($N_{\text{ch}}$), average transverse momentum ($\langle p_{\text{T}}\rangle$), initial eccentricity ($ε_{n}$), and flow harmonics ($v_{n}$), for different orientations of U+U collisions within the range of $0^{\circ} < θ< 120 ^{\circ}$ at $\sqrt{s_{\mathrm NN}} = 2.12$ GeV ($p_{\mathrm lab}$ = 500 MeV). Among the various collision configurations at this energy, the tip-tip scenario emerged with the highest average charged particle multiplicity, denoted as $\langle N_{\text{ch}} \rangle$. Notably, both the second and third-order eccentricities, $ε_{2,3}$, revealed intricate patterns as they varied with impact parameter across distinct configurations. The tip-tip configuration displayed the most pronounced magnitude of rapidity-odd directed flow ($v_{1}$), whereas the body-body configuration exhibited the least pronounced magnitude. Concerning elliptic flow ($v_{2}$) near mid-rapidity ($η< 1.0$), a negative sign is observed for all configurations except for the side-side exhibited a distinctly positive sign. Within the spectrum of configurations, the body-body scenario displayed the highest magnitude of $v_{2}$. For reaction plane correlated triangular flow ($v_{3}$), the body-body configuration emerged with the largest magnitude while the side-side exhibited the smallest magnitude. Our study seeks to establish a fundamental understanding of various U+U collision configurations in preparation for the forthcoming CEE experiment.

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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.

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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.

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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.

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arXiv2022

The high-density equation of state in heavy-ion collisions: Constraints from proton flow

A set of different equations of state is implemented in the molecular dynamics part of a non-equilibrium transport simulation (UrQMD) of heavy-ion collisions. It is shown how different flow observables are affected by the density dependence of the equation of state. In particular, the effects of a phase transition at high density are explored, including an expected reduction in mean $m_T$. We also show that an increase in $v_2$ is characteristic for a strong softening of the equation of state. The phase transitions with a low coexistence density, $n_{\mathrm{CE}}<4 n_0$, show a distinct minimum in the slope of the directed flow as a function of the beam energy, which would be a clear experimental signal. By comparing our results with experimental data, we can exclude any strong phase transition at densities below $4n_0$.

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arXiv2022

A Chiral Mean-Field Equation-of-State in UrQMD: Effects on the Heavy Ion Compression Stage

It is shown that the initial compression in central heavy ion collisions at beam energies of $E_\mathrm{lab}=1-10A$~GeV depends dominantly on the underlying equation of state and only marginally on the model used for the dynamical description. To do so, a procedure to incorporate any equation of state in the UrQMD transport model is introduced. In particular we compare the baryon density, temperature and pressure evolution as well as produced entropy in a relativistic ideal hydrodynamics approach and the UrQMD transport model, where the same equation of state is used in both approaches. Not only is the compression similar if the same equation of state is used in either dynamical model, but it also strongly depends on the actual equation of state. These results indicate that the equation of state can be studied with observables which are sensitive to the initial compression phase and maximum compression achieved in heavy ion collisions at these beam energies.

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arXiv2022

Transport Model Comparison Studies of Intermediate-Energy Heavy-Ion Collisions

Transport models are the main method to obtain physics information from low to relativistic-energy heavy-ion collisions. The Transport Model Evaluation Project (TMEP) has been pursued to test the robustness of transport model predictions in reaching consistent conclusions from the same type of physical model. Calculations under controlled conditions of physical input and set-up were performed with various participating codes. These included both calculations of nuclear matter in a box with periodic boundary conditions, and more realistic calculations of heavy-ion collisions. In this intermediate review, we summarize and discuss the present status of the project. We also provide condensed descriptions of the 26 participating codes, which contributed to some part of the project. These include the major codes in use today. We review the main results of the studies completed so far. They show, that in box calculations the differences between the codes can be well understood and a convergence of the results can be reached. These studies also highlight the systematic differences between the two families of transport codes, known as BUU and QMD type codes. However, when the codes were compared in full heavy-ion collisions using different physical models, as recently for pion production, they still yielded substantially different results. This calls for further comparisons of heavy-ion collisions with controlled models and of box comparisons of important ingredients, like momentum-dependent fields, which are currently underway. We often indicate improved strategies in performing transport simulations and thus provide guidance to code developers. Results of transport simulations of heavy-ion collisions from a given code will have more significance if the code can be validated against benchmark calculations such as the ones summarized in this review.

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