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

Abstract

Transport models are the main method to obtain physics information on the nuclear equation of state and in-medium properties of particles 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. To this end, 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, which test separately selected ingredients of a transport code, and more realistic calculations of heavy-ion collisions. Over the years, five studies have been performed within this project. 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. After a compact description of the underlying transport approaches, 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 under the names of Boltzmann-Uehling-Uhlenbeck (BUU) and Quantum Molecular Dynamics (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. Our evaluation studies often indicate improved strategies in performing transport simulations and thus can 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.

Studies of the equation-of-state of nuclear matter by heavy-ion collisions at intermediate energy in the multi-messenger era

“…Therefore, microscopic transport theory which does not require the assumption of local equilibrium was necessary for the interpretation of the observables. Many dynamical models suitable for HICs have been developed in the recent years and are continuously upgraded (see [49] for a review). Consequently, transport models are necessary to assess the properties of nuclear matter, and precise and systematic measurements of HICs varying system size and beam energy are necessary to constrain the various input parameters to the models, which cannot be deduced from first principles.…”

“…Attempts to study the symmetry energy using the single pion ratio have been faced with a strong model dependence of this observable [41][42][43][44] related to the differences in the treatment of in-medium pion production, propagation and, eventually, re-absorption in the available transport models, as discussed in the next sections. A collaborative effort on the theoretical side is on-going to clarify the situation [45][46][47][48][49]. At the same time, additional experimental measurements of this observable, from the SπIRIT Collaboration, have recently become available [35,50].…”

“…In general, past studies of the EoS employing HICs have revealed that theoretical predictions employing transport models often bear significant model dependence. Understanding the origin of these differences and their eventual resolution has been the goal of several past and present collaborations among transport theorists [49,74,75]. Given the small contribution of the symmetry energy to the full reaction pattern, it is hard to isolate its effects and to extract observables sensitive to it.…”

Event Generators for High-Energy Physics Experiments

“…The description of hadronic interactions at lower energies has a long history with a large variety of models and Monte Carlo programs [251]. These models describe low energy interactions via the excitation of resonances.…”

Horizons: Nuclear Astrophysics in the 2020s and Beyond

“…Additionally, heavy-ion collision experiments at these and other facilities such as NICA in Russia will directly probe dense matter to higher densities and larger neutron-proton asymmetries. It is crucial to better understand systematic uncertainties in the analysis of such experiments and to improve transport model simulations of heavy ion collisions [571]. These experimental constraints, together with progress in theoretical quantum many-body calculations of heavier nuclei, will allow us to improve and benchmark current and future models of nuclear interactions.…”