arXiv subjects

Massimo Papa

Publications and source records attributed to Massimo Papa.

12 recordsLinked to original sources

arXiv2024

Nuclear EoS with finite range interaction and the Constrained Dynamics

The effects of the many-body correlations with respect the MF description characterizing the Constrained Molecular Dynamics are discussed in the case of finite and zero range effective microscopic interactions. In particular, for the system $^{64}Ni+^{48}Ca$ fusion-fission cross-sections, IMF production and transversal flow have been evaluated in the framework of the CoMD model at different incident energies. The results obtained significantly depend on whether the effects of the above-mentioned correlations are taken into account in the corresponding EoS.

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arXiv2024

Comment on: "Investigating the time dependence of neutron-proton equilibration using molecular dynamics simulations" by A.Jedele et al: Phys Rev C 107, 024601 (2023)

In this paper, the authors discuss neutron-proton equilibration process induced on the 70Zn + 70Zn system at 35 MeV/nucleon, comparing experimental results with the Anti-symmetrized Molecular Dynamics and the Constrained Molecular Dynamics model simulations (COMD). The comment contains observations on the improper use of the COMD model, including some misinterpretations of the obtained results.

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arXiv2023

Nuclear Equation of State and many-body phase-space correlations in the Constrained Molecular Dynamics

Many-body correlations characterizing the Constrained Molecular Dynamics (CoMD)are analyzed in the case of finite and zero range effective microscopic interactions. The study begins by analyzing the case of infinite nuclear matter at zero temperature. A comparison with the predictions in the mean-field(MF) limit corresponding to different effective masses, highlights non-negligible differences regarding the produced Equation of State (EoS). A procedure is illustrated to determine the necessary corrections of the effective interaction parameters in the CoMD model so to reproduce the chosen EoS. The specific model calculations, the general feature of the discussed correlations gives a wider meaning to the resulting differences, which are in fact strongly related both to the Pauli principle constraint and to the localization effects related to wave-packets dynamics. Moving on to finite systems, preliminary results are shown in relation to the reaction mechanisms in the $^{64}Ni+^{48}Ca$ system described by the CoMD model. Therefore, the topic covered illustrates the effects produced by the correlations of the CoMD dynamics on the EoS and on some observables commonly studied in heavy ion collisions.

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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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arXiv2021

Temperature and Density Conditions for Alpha Clustering in Excited Self-Conjugate Nuclei

Starting from experimental studies on alpha-clustering in excited self-conjugate nuclei (from $^{16}$O to $^{28}$Si), temperature and density conditions for such a clustering are determined. Measured temperatures have been found in the range of 5.5 - 6.0 MeV whereas density values of 0.3 - 0.4 times the saturation density are deduced, i.e., 0.046 to 0.062 $fm^{-3}$. Such a density domain is also predicted by constrained self-consistent mean field calculations. These results constitute a benchmark for alpha clustering from self-conjugate nuclei in relation to descriptions of stellar evolution and supernovae.

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arXiv2021

Finite Range Interactions in Constrained Molecular Dynamics

Many-body correlations developed in the constrained molecular dynamics approach in the frame-work of the CoMD model are analyzed in the case of a finite range effective interaction. The study is performed in the case of infinite nuclear matter at zero temperature. A comparison with the predictions in the mean - field limit, which is taken as a reference case, highlights non-negligible differences concerning the produced Equation of State (EoS)for symmetric and asymmetric NM. A procedure is illustrated able to determine the necessary corrections of the effective interaction parameters in the CoMD model in such a way to reproduce the chosen reference EoS saturation properties. Even if the obtained numerical results are strictly valid for the CoMD model, the rather general feature of the discussed correlations gives a wider meaning to the obtained differences which are strongly related to the nucleon wave function space localization characterizing every semi-classical quantum molecular dynamics approach. Finally, the relevance of the constraint on the primary light cluster production is shortly illustrated in an appendix section.

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arXiv2017

Comparison of heavy-ion transport simulations: Collision integral in a box

Simulations by transport codes are indispensable to extract valuable physics information from heavy ion collisions. In order to understand the origins of discrepancies between different widely used transport codes, we compare 15 such codes under controlled conditions of a system confined to a box with periodic boundary, initialized with Fermi-Dirac distributions at saturation density and temperatures of either 0 or 5 MeV. In such calculations, one is able to check separately the different ingredients of a transport code. In this second publication of the code evaluation project, we only consider the two-body collision term, i.e. we perform cascade calculations. When the Pauli blocking is artificially suppressed, the collision rates are found to be consistent for most codes (to within $1\%$ or better) with analytical results, or completely controlled results of a basic cascade code after eliminating the correlations within the same pair of colliding particles. In calculations with active Pauli blocking, the blocking probability was found to deviate from the expected reference values. The reason is found in substantial phase-space fluctuations and smearing tied to numerical algorithms and model assumptions in the representation of phase space. This results in the reduction of the blocking probability in most transport codes, so that the simulated system gradually evolves away from the Fermi-Dirac towards a Boltzmann distribution. As a result of this investigation, we are able to make judgements about the most effective strategies in transport simulations for determining the collision probabilities and the Pauli blocking. Investigation in a similar vein of other ingredients in transport calculations, like the mean field propagation or the production of nucleon resonances and mesons, will be discussed in the future publications.

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arXiv2016

Understanding transport simulations of heavy-ion collisions at 100 and 400 AMeV: Comparison of heavy ion transport codes under controlled conditions

Transport simulations are very valuable for extracting physics information from heavy-ion collision experiments. With the emergence of many different transport codes in recent years, it becomes important to estimate their robustness in extracting physics information from experiments. We report on the results of a transport code comparison project. 18 commonly used transport codes were included in this comparison: 9 Boltzmann-Uehling-Uhlenbeck-type codes and 9 Quantum-Molecular-Dynamics-type codes. These codes have been required to simulate Au+Au collisions using the same physics input for mean fields and for in-medium nucleon-nucleon cross sections, as well as the same initialization set-up, the impact parameter, and other calculational parameters at 100 and 400 AMeV incident energy. Among the codes we compare one-body observables such as rapidity and transverse flow distributions. We also monitor non-observables such as the initialization of the internal states of colliding nuclei and their stability, the collision rates and the Pauli blocking. We find that not completely identical initializations constitute partly for different evolutions. Different strategies to determine the collision probabilities, and to enforce the Pauli blocking, also produce considerably different results. There is a substantial spread in the predictions for the observables, which is much smaller at the higher incident energy. We quantify the uncertainties in the collective flow resulting from the simulation alone as about $30\%$ at 100 AMeV and $13\%$ at 400 AMeV, respectively. We propose further steps within the code comparison project to test the different aspects of transport simulations in a box calculation of infinite nuclear matter. This should, in particular, improve the robustness of transport model predictions at lower incident energies where abundant amounts of data are available.

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arXiv2012

Many-body correlations in Semiclassical Molecular Dynamics and Skyrme interaction

Constraint Molecular dynamics CoMD calculations have been performed for asymmetric nuclear matter (NM) by using a simple effective interactions of the Skyrme type. The set of parameter values reproducing common accepted saturation properties of nuclear matter have been obtained for different degree of stiffness characterizing the iso-vectorial potential density dependence. A comparison with results obtained in the limit of the Semi-Classical Mean Field approximation using the same kind of interaction put in evidence the role played by the many-body correlations in to explain the noticeable differences obtained in the parameter values in the two cases. Even if from a numerical point of view the obtained results are strictly valid for the CoMD model, some rather general feature of the discussed correlations can give a wider meaning to the obtained differences being strongly related to the spacial correlations generated in the semiclassical wave packets dynamics.

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arXiv2012

Many-body correlations in Semiclassical Molecular Dynamics and Skyrme forces for symmetric Nuclear Matter

Constraint Molecular dynamics CoMD calculations have been performed for symmetric nuclear matter (NM) by using a simple effective interactions of the Skyrme type. The set of parameter values reproducing common accepted saturation properties of nuclear matter have been obtained for different degree of stiffness characterizing the iso-vectorial potential density dependence. A comparison with results obtained in the limit of the Semi-Classical Mean Field approximation performed using the same kind of interaction put in evidence the role played by the many-body correlations present in the model explaining also the noticeable differences obtained in the parameter values in the two cases

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arXiv2001

Vlasov calculations of nuclear ground states

We describe some properties of nuclear ground states using a microscopic Vlasov approach. We propose a new method to find the energy minimum of the system enforcing the fermionic nature of the system. Calculations are performed for two different parametrization of the mean field: a simple momentum independent Skyrme potential and a momentum dependent one as proposed by Gale, Bertsch and Das Gupta. We show that the binding energies and radii of nuclei from $^{16}$O to $^{208}$Pb are reasonably reproduced by opportunely tuning the surface term. We also made some calculations for neutron rich/poor nuclei.

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arXiv2001

Formation and decay of super heavy systems

We investigate the formation and the decay of heavy systems which are above the fission barrier. By using a microscopic simulation of constraint molecular dynamics (CoMD) on Au+Au collision, we observe composite states stay for very long time before decaying by fission.

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