Quantum Molecular Dynamics Model Research Articles

Correlation between magnetic field and nuclear stopping in different rapidity segments during heavy ion collisions

The mid-rapidity zone is a region of crucial scientific interest during heavy ion collisions as it offers deep insight into the complex dynamics and properties of extremely dense nuclear matter. In the framework of the isospin-dependent quantum molecular dynamics model, the mid-rapidity zone (–0.6 ≤ Y c.m./Y beam ≤ 0.6) is segregated into three different regions to discover the intricate correlation between nuclear stopping (〈R p 〉) and the magnetic field eBy(0,0,0) generated due to the nucleons from these rapidity segments. Moreover, study of the time and space evolution of eBy(0,0,0) has been carried out for all the three rapidity bins. The maximum magnetic field intensity eBy(0,0,0)max is observed in the outwardmost region of the mid-rapidity zone due to less nuclear stopping and fewer nucleon–nucleon collisions. Additionally, a fruitful correlation has also been observed between eccentricity and nuclear stopping.

Momentum correlation of light nuclei in Au + Au collisions at [formula omitted] GeV

Within the Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) coupled with nucleon coalescence model and Mini-Spanning-Tree model, the yields of light nuclei have been stimulated in Au + Au collisions over an energy range of sNN=2.0∼7.7GeV and the momentum correlation functions of light nuclei pairs have been calculated by both the Lednický-Lyuboshitz and the Correlation After Burner methods. We compare the yields of light nuclei and their momentum correlation functions at midrapidity in this energy region with experimental data. It is found that there are differences between the results of the two models, and the coalescence method seems less valid at low collision energy. Furthermore, both the peak values of proton-proton correlation functions and the transition point of elliptic flows from out-of-plane to in-plane emission show a turning point around 3-4 GeV, which suggests that there is a relation between momentum correlation function and collective flow of particles.

Properties of collective flow and pion production in intermediate-energy heavy-ion collisions with a relativistic quantum molecular dynamics model

Properties of collective flow and pion production in intermediate-energy heavy-ion collisions with a relativistic quantum molecular dynamics model

3-dimensional flow analysis: A novel tool to study the collision geometry and the equation-of-state

We propose to extend the commonly known flow analysis in the transverse px-py plane to novel flow coefficients based on the angular distribution in the px-pz and py-pz planes. The new flow coefficients, called un and wn (in addition to vn), turn out to be also highly sensitive to the nuclear Equation-of-State and can be used to explore the EoS in more detail than is possible using only vn. As an example to quantify the effect of the EoS, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model is used to investigate 20-30% central Au+Au collisions at E=lab1.23AGeV.

Uncertainties of critical temperatures based on higher-order fluctuations of the largest fragment charge* *Supported by the National Natural Science Foundation of China (11875328, 12075327), the Key Laboratory of Nuclear Data foundation (JCKY2022201C157), and the Fundamental Research Funds for the Central Universities, Sun Yat-sen University (22lgqb39)

The new signature of liquid-gas phase transition has been well indicated by the higher-order fluctuations of the largest fragment charge, but the uncertainties of critical temperatures based on this signature have not been revealed. This study extracts the critical temperatures of liquid-gas phase transition in nuclear reactions and investigates their uncertainties. Utilizing the isospin-dependent quantum molecular dynamics model in conjunction with the statistical model GEMINI enables us to describe the dynamical path from the initial to the final state. An isotope thermometer and a quantum fluctuation thermometer are employed to extract the nuclear temperature. The higher-order fluctuations of the largest fragment charge and critical temperatures are studied in 124Sn + 120Sn collisions ranging from 400 to 1000 MeV/nucleon and 124Sn + AZ collisions at 600 MeV/nucleon. Observations revealed that the pseudo-critical point is robustly indicated by the higher-order fluctuations of the largest fragment charge. The critical temperatures extracted by the isotope thermometer are relatively consistent, with an uncertainty of 15%, while those obtained by the quantum fluctuation thermometer are heavily influenced by the incident energy and mass number of target nuclei. The excitation energy and bound charge are used for event-sorting. These two ensembles represent the statistical properties of the initial and final states of the system, respectively. The initial-final correlations of statistical properties might lead to two phenomena. First, the size distribution of the largest fragment at the pseudo-critical point based on the ensemble is wide, while that based on ensemble exhibits bimodality, which is a typical characteristic in the liquid-gas coexistence of a finite system. Second, the temperature at the pseudo-critical point based on the ensemble is higher than that based on the ensemble. Furthermore, the projectile-like system exhibits a significant dynamical effect in its evolution path from the initial to final state, closely associated with the fluctuation of critical temperature.

Role of equation of state and clusterization algorithms on nuclear vaporization

By using the Quantum Molecular Dynamics (QMD) model, a study on the nuclear vaporization in [Formula: see text] collision is presented for different nuclear equations of state along with a systematic comparison of different clusterization methods based on simple spatial correlations, spatial-momentum correlations, mass dependent binding energy cuts and Simulated Annealing Clusterization Algorithm (SACA). The effect of different nuclear equations of state i.e., Soft, Hard and Soft with Momentum Dependent (SMD) interactions on the energy of onset of vaporization for [Formula: see text] collisions is predicted by investigating gas/liquid content and probability of vaporization versus incident energy behavior. These two observables probe the critical point of nuclear vaporization very well. Further outcome of different clusterization algorithms on the energy of onset of nuclear vaporization is also probed and a comparison of calculations with the experimental data for [Formula: see text] and [Formula: see text] collisions is carried out for different clusterization algorithms.

Charge exchange processes during carbon ion fragmentation at an energy of 300 MeV/nucleon: comparison with models of ion-ion interactions

Experimental data are presented on the differential cross sections to produce 11Be, 12Be, and 12B isotopes at the carbon beam fragmentation with an energy of 300 MeV/nucleon in the FRAGM experiment conducted based on the ITEP-TWAС multipurpose accelerator complex. The obtained experimental data are compared with the predictions of the models of ion-ion interactions: binary cascade and quantum molecular dynamics model.

Cumulants of net-strangeness multiplicity distributions at energies available at the BNL Relativistic Heavy Ion Collider

The higher-order cumulants of net-proton number, net-charge, and net-strangeness multiplicity distributions are widely studied to search for the quantum-chromodynamics critical point and extract the chemical freeze-out parameters in heavy-ion collisions. In this context, the event-by-event fluctuations of the net-strangeness multiplicity distributions play important roles in extracting the chemical freeze-out parameter in the strangeness sector. Due to having difficulties in detecting all strange hadrons event by event, the kaon ($K$) and lambda ($\Lambda$) particles serve as a proxy for the strangeness-related observables in heavy-ion collisions. We have studied the net-$K$, net-$\Lambda$, and net-($K$ + $\Lambda$) multiplicity distributions and calculated their different order of cumulants using the ultrarelativistic quantum molecular dynamics model and hadron resonance gas calculation. To adequately account for the net-strangeness cumulants, it has been found that the inclusion of resonance decay contributions in $K$ and $\Lambda$ is necessary.

Probing onset of nuclear vaporisation in heavy-ion collisions

The onset of nuclear vaporisation in heavy-ion collisions is examined in the present study. For this, we perform quantum molecular dynamics (QMD) model calculations supplemented with a clusterisation algorithm for fragment identification, namely the Simulated Annealing Clusterization algorithm (SACA) approach. Our results with the very sophisticated SACA method show a good agreement with the experimental findings of vaporisation (predicted by the asymptotic behaviour of the average fragment charge) in 16O + 80Br and 16O + 107Ag collisions. Further, we predicted the energy of the onset of vaporisation for 40Ca + 40Ca, 84Kr + 84Kr, 132Xe + 132Xe and 197Au + 197Au collisions by investigating the gas/liquid content and probability of vaporisation (and its derivative) versus the incident energy behaviour. These two observables probe the critical point of nuclear vaporisation in an extremely sophisticated manner, relative to an average fragment charge. Our findings on these two novel variables to predict the energy of the onset of vaporisation is verifiable in experiments. The influence of the colliding geometry as well as the role of Coulomb interactions are also studied to understand the system size effects on nuclear vaporisation.

Forward-backward multiplicity correlation and fluctuation in 197Au + 197Au collisions at fixed target Elab = 10A, 20A, 30A and 40A GeV in the UrQMD model

We use the ultra-relativistic quantum molecular dynamics (UrQMD) model to study the multiplicity correlations and fluctuations of charged hadrons emitted in the forward and backward rapidity hemispheres in 197Au + 197Au collisions at four different incident beam energies, namely Elab=10A,20A, 30A and 40A GeV. Dependence of the correlation and fluctuation parameters on the incident beam energy, collision centrality, rapidity gap and rapidity window size are examined. Both short and long-ranged correlations contribute to our UrQMD simulated results. In the framework of the UrQMD model the short-ranged correlations can be interpreted in terms of resonance decays and/or cluster formation. The long-ranged correlations on the other hand, perhaps originate from multiple rescattering suffered by the hadrons and decay of longitudinally stretched strings in the rapidity space. Our simulation based analysis can be used to set a reference base-line for any future experimental study on the forward-backward multiplicity correlation for the colliding system and collision energies involved in the present case.

Probing the Neutron Skin of Unstable Nuclei with Heavy-Ion Collisions

To improve the constraints of symmetry energy at subsaturation density, measuring and accumulating more neutron skin data for neutron-rich unstable nuclei is naturally required. Aiming to probe the neutron skin of unstable nuclei by using low-intermediate-energy heavy-ion collisions, we develop a new version of an improved quantum molecular dynamics model, in which the neutron skin of the initial nucleus and the mean-field potential in nucleon propagation are consistently treated. Our calculations show that the three observables, such as the cross-sections of the primary projectile-like residues with A>100 (σA>100), the difference of σA>100 between 132Sn + 124Sn and 124Sn + 124Sn systems (δσA>100), and the neutron-to-proton yield ratio (R(n/p)) in the transverse direction, could be used to measure the neutron skin of the unstable nuclei and to constrain the slope of the symmetry energy in the future.

Simulation of collective flow of protons and deuterons in Au+Au collisions at Ebeam=1.23A GeV with the isospin-dependent quantum molecular dynamics model

Collective flows of protons and deuterons for $\text{Au}+\text{Au}$ collisions at beam energy ${E}_{\text{beam}}=1.23A \mathrm{GeV}$ were simulated by an isospin-dependent quantum molecular dynamics model. Two coalescence models, namely, naive coalescence and dynamical coalescence models, are compared for the formation of deuterons. After a reasonable agreement of the proton and deuteron rapidity spectra with the high acceptance dielectron spectrometer data is obtained, we apply an event-plane method to calculate the first four order collective flow coefficients and the ratios of $\ensuremath{\langle}{v}_{4}\ensuremath{\rangle}/{\ensuremath{\langle}{v}_{2}\ensuremath{\rangle}}^{2}$ and $\ensuremath{\langle}{v}_{3}\ensuremath{\rangle}/(\ensuremath{\langle}{v}_{1}\ensuremath{\rangle}\ensuremath{\langle}{v}_{2}\ensuremath{\rangle})$ and observe the scaling of the number of constituent nucleons between protons and deuterons. In addition, the dependence of ${\ensuremath{\varepsilon}}_{n}$ versus $\ensuremath{\langle}{v}_{n}\ensuremath{\rangle}$ and the ratio $\ensuremath{\langle}{v}_{n}\ensuremath{\rangle}/{\ensuremath{\varepsilon}}_{n}$ on the centrality is obtained. Finally, the Pearson coefficients $\text{corr}({v}_{n},{v}_{m})$ between the first four harmonic flows for protons and deuterons are studied as a function of rapidity and centrality.

Effects of sequential decay on collective flows and nuclear stopping power in heavy-ion collisions at intermediate energies

In this study, the rapidity distribution, collective flows, and nuclear stopping power in ^{197}Au+^{197}Au collisions at intermediate energies were investigated using the ultrarelativistic quantum molecular dynamics (UrQMD) model with GEMINI++ code. The UrQMD model was adopted to simulate the dynamic evolution of heavy-ion collisions, whereas the GEMINI++ code was used to simulate the decay of primary fragments produced by UrQMD. The calculated results were compared with the INDRA and FOPI experimental data. It was found that the rapidity distribution, collective flows, and nuclear stopping power were affected to a certain extent by the decay of primary fragments, especially at lower beam energies. Furthermore, the experimental data of the collective flows and nuclear stopping power at the investigated beam energies were better reproduced when the sequential decay effect was included.

Production of magnetic field due to heavy ion collisions around transition energy

Isospin quantum molecular dynamics (IQMD) model is a reliable computational tool for the study of various phenomena (multi fragmentation, anisotropic flow, nuclear stopping) from low to intermediate heavy ion collisions (HICs). Here, simulation has been carried out for the magnetic field generated during non-central HICs using the IQMD model. The effect of various parameters, such as centrality, angular momentum, rapidity and incident energy has been thoroughly investigated on the magnetic field with the evolution of time and space. It has been observed that the rapidity bin significantly impacts the magnitude of the magnetic field in different directions. Furthermore, the magnetic field produced during HIC leads to a notable impact on the in-plane momentum of the proton and neutron.

The description of dynamical fission process using improved quantum molecular dynamics model incorporated with microscopic potential energy surface

A new method, namely the improved quantum molecular dynamics model incorporated with the microscopic potential energy surface, is proposed to simulate the dynamical evolution from saddle to scission point and to calculate the isotope yields of fission fragments in thermal neutron-induced fission of actinide nuclei near 236U. With this approach, the shell and pairing effects are introduced by the microscopic potential energy, while the isospin effect, the dynamical effects of fluctuations and correlations in fission process are automatically included in the improved quantum molecular dynamics model. These two aspects eventually influence the productions of fission fragments. The calculated both charge and mass distributions of fission fragments for a series of actinide nuclei are in overall agreement with the evaluated data from JEFF-3.3 and ENDF/B-VIII.0. The isotope distributions of fission fragments in the fission of 236U reproduce the data reasonable well, from which we extract the relationship between peak mass number of the isotope distributions and the charge number of fission fragments. We find that this relationship deviates linear relation slightly at certain cases, which indicates the isospin nonequilibrium effect. By tracking back to the fission process on the two dimension plane of elongation and mass asymmetry of prefragments, we find that the necked-in shape and the separation of two parts happen much faster for the 132Sn than 128,136Sn in the time evolution of the distance R between the centers of two parts of the fissioning system. The events producing 128,132,136Sn and 136,140,144Xe present two kinds of average fission paths with bypassing and going through the minimum-energy valley of the microscopic potential energy surface, respectively.

Dynamics of charge equilibration and effects on producing neutron-rich isotopes around N=126 in multinucleon transfer reactions

The dynamics of the charge equilibration (CE) and the effects on the production of the neutron-rich isotopes around $N = 126$ in multinucleon transfer reactions are still not well understood. In this Letter, we investigate the mechanism of the CE from different viewpoints by using the extended version of the dinuclear system model (DNS-sysu) and the improved quantum molecular dynamics (ImQMD) model. From the macroscopic and microscopic dynamical viewpoints, we find incomplete CE for the mass asymmetry reaction systems even in the very deep collisions, and the behavior of "inverse CE" that the tendency of the fragments is away from the $N/Z$ value of the compound system in the reaction $^{140}$Xe + $^{198}$Pt. Unlike the slow process presented in the ImQMD model, the behavior of fast equilibration with the characteristic time $\sim$ 0.52 zs is obtained based on the DNS-sysu model, which is consistent with the experimental data. By performing the systematic calculation, the correlation between the CE and the mass asymmetry of the reaction systems is clarified, which not only accounts for the observed intriguing phenomena of the CE but also provides essential information for producing the neutron-rich isotopes around $N = 126$.

Elliptic flow in heavy-ion collisions at intermediate energy: The role of impact parameter, mean field potential, and collision term

Within the ultrarelativistic quantum molecular dynamics (UrQMD) model, by reversely tracing nucleons that are finally emitted at mid-rapidity (|y0| < 0.1) in the entire reaction process, the time evolution of elliptic flow (v2) of these traced nucleons is studied in Au+Au collisions at beam energy of 0.4 GeV/nucleon with different impact parameters. The initial value of v2 is positive and increases with impact parameter, it then decreases as time passes and tends to saturate at a negative value. It is found that nucleon-nucleon collisions always suppress the value of v2 (enhance the out-of-plane emission), while the nuclear mean field potential effect is more complex, which depends on both impact parameter and reaction time. The most relevant density probed by v2 of nucleons at mid-rapidity is found to be ∼ 60% of the maximum density reached during the collisions.

Light hypernuclei in heavy-ion collisions

Prediction for hyper nuclei multiplicities from GSI to LHC energies from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model combined with a final state coalescence approach is presented and compared to the thermal model. The influence of the coalescence radius on the collision energy and centrality dependence of the Λ3H/Λ ratio is discussed.

Onset of pressure gradient on collective flow through balance and transition geometry

The impression of mean field and spectator matter has been studied through the collision geometry dependence of reduced flow and elliptical flow in heavy-ion collisions (HICs) at intermediate energies using Isospindependent Quantum Molecular Dynamics (IQMD) model for the reaction of $$_{20}^{50}{\rm{Cu + }}_{20}^{50}{\rm{and}}_{79}^{197}{\rm{Au + }}_{79}^{197}{\rm{Au}}$$ at incident energy between 50 MeV/nucleon and 450 MeV/nucleon. The collision geometry at which the reduced flow and elliptical flow changes its sign is termed as balance geometry $$({\hat b_{bal}})$$ and transition geometry $$({\hat b_{trans}})$$ of flow, respectively. Our study reveals that $$({\hat b_{bal}})$$ and $$({\hat b_{trans}})$$ increases with increase in incident energy. In addition to this, the effect of nuclear charge radius has also been studied by using the isospin-independent as well dependent nuclear charge radii parameterizations.

Intermittency of charged particles in the hybridUrQMD+CMCmodel at energies available at the BNL Relativistic Heavy Ion Collider

Within the framework of intermittency analysis, a search for critical fluctuations is ongoing to locate the possible critical point in the quantum chromodynamics phase diagram. In this study, self-similar critical fluctuations from a critical Monte Carlo (CMC) model have been incorporated into the cascade ultrarelativistic quantum molecular dynamics (UrQMD) model. This hybrid UrQMD+CMC model exhibits a clear power-law behavior of scaled factorial moment for charged particles in Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 7.7-200 GeV. By comparing the UrQMD+CMC model results with those from the STAR experiment, it is found that the value of a calculated scaling exponent falls in the range of the experimental measurement when 1-2 \% signal of intermittency fluctuations is added into the UrQMD sample.