arXiv subjects

Akira Ohnishi

Publications and source records attributed to Akira Ohnishi.

At least 19 recordsLinked to original sources

arXiv2026

Theoretical study of the $Ξα$ correlation function

We study $Ξ$-$^4{\mathrm{He}}$ ($α$) momentum correlation functions in the high-energy nuclear collisions to investigate the nature of the $ΞN$ interactions. We employ the folding $Ξα$ potential based on the lattice QCD $ΞN$ interactions to compute the correlation function. The $Ξα$ potential supports a Coulomb-assisted bound state ${}^5_Ξ\mathrm{H}$ in the $Ξ^-α$ channel, while the $Ξ^0α$ channel is unbound. To examine the sensitivity of the correlation function to the nature of the $Ξα$ interaction, we vary the potential strength simulating stronger and weaker interactions. The result of the correlation function is sensitive to the existence of the bound state in the $Ξ^0 α$ channel, and the characteristic behavior of the bound state remains also in the $Ξ^-α$ correlation with the Coulomb interaction. The effect of the repulsive core of the $Ξα$ potential can be found in the correlation from the small source as the distinctive dip in the intermediate momentum region.

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arXiv2024

Theoretical study on $Λα$ and $Ξα$ correlation functions

We examine the $Λ$-${}^4\mathrm{He}$ ($α$) and $Ξα$ momentum correlation in high-energy collisions to further elucidate the properties of the hyperon-nucleon interactions. For the $Λα$ system, we compare $Λα$ potential models with different strengths at short range. We find that the difference among the models is visible in the momentum correlation from a small-size source. This indicates that the $Λα$ momentum correlation can constrain the property of the $ΛN$ interaction at short range, which plays an essential role in dense nuclear matter. For the $Ξα$ system, we employ the folding $Ξα$ potential based on the lattice QCD $ΞN$ interactions. The $Ξα$ potential supports a Coulomb assisted bound state of ${}^5_Ξ\mathrm{H}$ in the $Ξ^-α$ channel, while the $Ξ^0α$ channel is unbound. To examine the sensitivity of the correlation function to the nature of the $Ξα$ potential, we vary the potential strength simulating stronger and weaker interactions. The result of the correlation function clearly reflects the bound state nature in the $Ξ^-α$ correlation.

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arXiv2024

Femtoscopic study of the $Λα$ interaction

We examine the $Λ$-${}^4\mathrm{He}$ ($α$) momentum correlation in high-energy collisions to elucidate the interaction between Lambdas ($Λ$) and nucleons ($N$). We compare phenomenological $Λα$ potentials with different strengths at short range. In addition to the conventional Gaussian-type potentials, we construct the $Λα$ potentials by substituting the nucleon density distribution in $α$ into the Skyrme-type $Λ$ potentials. We find that the dependence on the employed potential models is visible in the correlation functions from a small-size source. This indicates that the $Λα$ momentum correlation could constrain the property of the $ΛN$ interaction at high densities, which is expected to play an essential role in dense nuclear matter. Also, we verify that the Lednicky-Lyuboshits formula can yield erroneous results for a small-size source with a potential which has a large interaction range, like the $Λα$ system.

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arXiv2024

Estimation of electric field in intermediate-energy heavy-ion collisions

We estimate the spacetime profile of the electric field in head-on heavy-ion collisions at intermediate collision energies $\sqrt{s_{\rm NN}} = {\mathcal O}(3 \;-\; 10\;{\rm GeV})$. Using a hadronic cascade model (JAM; Jet AA Microscopic transport model), we numerically demonstrate that the produced field has strength $eE = {\mathcal O}((30 \;-\; 60\;{\rm MeV})^2)$, which is supercritical to the Schwinger limit of QED and is non-negligibly large compared even to the hadron/QCD scale, and survives for a long time $τ= {\mathcal O}(10\;{\rm fm}/c)$ due to the baryon stopping. We show that the produced field is nonperturbatively strong in the sense that the nonperturbativity parameters (e.g., the Keldysh parameter) are sufficiently large. This is in contrast to high-energy collisions $\sqrt{s_{\rm NN}} \gtrsim 100 \;{\rm GeV}$, where the field is extremely short-lived and hence is perturbative. Our results imply that the electromagnetic field may have phenomenological impacts on hadronic/QCD processes in intermediate-energy heavy-ion collisions and that heavy-ion collisions can be used as a new tool to explore strong-field physics in the nonperturbative regime.

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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

Application of the path optimization method to a discrete spin system

The path optimization method, which is proposed to control the sign problem in quantum field theories with continuous degrees of freedom by machine learning, is applied to a spin model with discrete degrees of freedom. The path optimization method is applied by replacing the spins with dynamical variables via the Hubbard-Stratonovich transformation, and the sum with the integral. The one-dimensional (Lenz-)Ising model with a complex coupling constant is used as a laboratory for the sign problem in the spin model. The average phase factor is enhanced by the path optimization method, indicating that the method can weaken the sign problem. Our result reproduces the analytic values with controlled statistical errors.

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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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arXiv2023

Improving efficiency of the path optimization method for a gauge theory

We investigate efficiency of a gauge-covariant neural network and an approximation of the Jacobian in optimizing the complexified integration path toward evading the sign problem in lattice field theories. For the construction of the complexified integration path, we employ the path optimization method. The $2$-dimensional $\text{U}(1)$ gauge theory with the complex gauge coupling constant is used as a laboratory to evaluate the efficiency. It is found that the gauge-covariant neural network, which is composed of the Stout-like smearing, can enhance the average phase factor, as the gauge-invariant input does. For the approximation of the Jacobian, we test the most drastic case in which we perfectly drop the Jacobian during the learning process. It reduces the numerical cost of the Jacobian calculation from ${\cal O}(N^3)$ to ${\cal O}(1)$, where $N$ means the number of degrees of freedom of the theory. The path optimization using this Jacobian approximation still enhances the average phase factor at expense of a slight increase of the statistical error.

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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

Entropy production in longitudinally expanding Yang-Mills field with use of Husimi function$-$semiclassical approximation

We investigate the possible thermalization process of the highly occupied and weakly coupled Yang-Mills fields expanding along the beam axis through an evaluation of the entropy, particle number, and pressure anisotropy. The time evolution of the system is calculated by solving the equation of motion for the Wigner function in the semiclassical approximation with initial conditions mimicking the glasma. For the evaluation of the entropy, we adopt the Husimi-Wehrl (HW) entropy, which is obtained by using the Husimi function, a positive semidefinite quantum distribution function given by smearing the Wigner function. By numerical calculations at $g=0.1$ and $0.2$, the entropy production is found to occur together with the particle creation in two distinct stages: In the first stage, the particle number and the entropy at low longitudinal momenta grow rapidly. In the second stage, the particle number and the entropy of higher longitudinal momentum modes show slower increase. The pressure anisotropy remains in our simulation and implies that the system is still out-of-equilibrium.

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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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arXiv2022

Femtoscopic study on $DD^$ and $D\bar{D}^$ interactions for $T_{cc}$ and $X(3872)$

We investigate $DD^*$ and $D\bar{D}^*$ momentum correlations in high-energy collisions to elucidate the nature of $T_{cc}$ and $X(3872)$ exotic hadrons. Single range Gaussian potentials with the channel couplings to the isospin partners are constructed based on the empirical data. The momentum correlation functions of the $D^0D^{*+}$, $D^+D^{*0}$, $D^0\bar{D}^{*0}$, and $D^+D^{*-}$ pairs are computed with including the coupled-channel effects. We discuss how the nature of the exotic states are reflected in the behaviors of the correlation results.

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arXiv2022

Mean-field update in the JAM microscopic model: Mean-field effects on collective flow in high-energy heavy-ion collisions at $\sqrt{s_{NN}}=2-20$ GeV energies

The beam energy dependence of the directed flow is a sensitive probe for the properties of strongly interacting matter. We consider different implementations of momentum-dependent hadronic mean fields in the relativistic quantum molecular dynamics (RQMD) framework. First, Lorentz scalar implementation of a Skyrme type potential is examined. Then, full implementation of the Skyrme type potential as a Lorentz vector in the RQMD approach is proposed. We find that scalar implementation of the Skyrme force is too weak to generate repulsion explaining observed data of sideward flows at $\sqrt{s_{NN}}<10$ GeV, while vector implementation gives collective flows compatible with the data for a wide range of beam energies $2.7 <\sqrt{s_{NN}}<20$ GeV. We show that our approach reproduces the negative proton directed flow at $\sqrt{s_{NN}}>10$ GeV discovered by experiments. We discuss the dynamical generation mechanisms of the directed flow within a conventional hadronic mean field. A positive slope of proton directed flow is generated predominantly during compression stages of heavy-ion collisions by the strong repulsive interaction due to high baryon densities. In contrast, in the expansion stages of the collision, the negative directed flow is generated more strongly than the positive one by the tilted expansion and shadowing by the spectator matter. At lower collision energies $\sqrt{s_{NN}}<10$ GeV, the positive flow wins against the negative flow because of a long compression time. On the other hand, at higher energies $\sqrt{s_{NN}}>10$ GeV, negative flow wins because of shorter compression time and longer expansion time. A transition beam energy from positive to negative flow is highly sensitive to the strength of the interaction.

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arXiv2022

Gauge invariant input to neural network for path optimization method

We investigate the efficiency of a gauge invariant input to a neural network for the path optimization method. While the path optimization with a completely gauge-fixed link-variable input has successfully tamed the sign problem in a simple gauge theory, the optimization does not work well when the gauge degrees of freedom remain. We propose to employ a gauge invariant input, such as plaquette, to overcome this problem. The efficiency of the gauge invariant input to the neural network is evaluated for the 2-dimensional $U(1)$ gauge theory with a complex coupling. The average phase factor is significantly enhanced by the path optimization with the plaquette input, indicating good control of the sign problem. It opens a possibility that the path optimization is available to complicated gauge theories, including Quantum Chromodynamics, in a realistic setup.

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arXiv2021

Effect of deuteron breakup on the deuteron-$Ξ$ correlation function

The hadron-deuteron correlation function has attracted many interests as a potential method to access the three-hadron interactions. However, the weakly-bound nature of deuteron has not been considered in the preceding studies. In this study, the breakup effect of deuteron on the deuteron-$Ξ^-$ ($d$-$Ξ^-$) correlation function $C_{dΞ^-}$ is investigated. The $d$-$Ξ^-$ scattering is described by a nucleon-nucleon-$Ξ$ three-body reaction model. The continuum-discretized coupled-channels method, which is a fully quantum-mechanical and non-perturbative reaction model, is adopted. $C_{dΞ^-}$ turns out to be sensitive to the strong interaction and enhanced by the deuteron breakup effect by 6--8 % for the $d$-$Ξ^-$ relative momentum below about 70 MeV/$c$. Low-lying neutron-neutron continuum states are responsible for this enhancement. Within the adopted model, the deuteron breakup effect on $C_{dΞ^-}$ is found to be appreciable but not very significant. Except for the enhancement by several percent, studies on $C_{dΞ^-}$ without the deuteron breakup effect can be justified.

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arXiv2020

Replica evolution of classical field in 4+1 dimensional spacetime toward real time dynamics of quantum field

Real-time evolution of replicas of classical field is proposed as an approximate simulator of real-time quantum field dynamics at finite temperatures. We consider $N$ classical field configurations dubbed as replicas which interact with each other via the $τ$-derivative terms and evolve with the classical equation of motion. The partition function of replicas is found to be proportional to that of quantum field in the imaginary time formalism. As the replica index $τ$ can be regarded as the imaginary time index, the replica evolution is technically the same as the molecular dynamics part of the hybrid Monte-Carlo sampling and the replica configurations should reproduce the correct quantum equilibrium distribution after the long-time evolution. At the same time, evolution of the replica-index average of field variables is described by the classical equation of motion when the fluctuations are small. In order to examine the real-time propagation properties of replicas, we first discuss replica evolution in quantum mechanics. Statistical averages of observables are precisely obtained by the initial condition average of replica evolution, and the time evolution of the unequal-time correlation function, $\langle x(t) x(t')\rangle$, in a harmonic oscillator is also described well by the replica evolution in the range $T/ω> 0.5$. Next, we examine the statistical and dynamical properties of the $ϕ^4$ theory in the 4+1 dimensional spacetime, which contains three spatial, one replica index or the imaginary time, and one real-time. We note that the Rayleigh-Jeans divergence can be removed in replica evolution with $N \geq 2$ when the mass counterterm is taken into account. We also find that the thermal mass obtained from the unequal-time correlation function at zero momentum grows as a function of the coupling as in the perturbative estimate in the small coupling region.

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