Understanding transport simulations of heavy-ion collisions at100Aand400AMeV: Comparison of heavy-ion transport codes under controlled conditions

Abstract

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-MolecularDynamics-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.

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

“…It continued through dedicated workshops at the Shanghai Jiao Tong University, China [12], and at the Facility for Rare Isotope Beams/National Superconducting Cyclotron Laboratory (FRIB/NSCL) in East Lansing, USA [13], in addition to various smaller satellite workshops tied to larger meetings. The final result of the Shanghai workshop was a comparison of 19 widely used codes in the field, in simulations of Au + Au collisions at incident energies of 100 and 400 A MeV, for specified input mean field and cross sections, as well as specified impact parameters and initialization details.…”

“…The compared aspects included the stability of the initialized nuclei, the distributions of the collision probabilities in time and energy, the efficiency of Pauli blocking for the final states, and predicted flow observables. The outcome of these investigations was published in 2016 [12]. An attempt was made to quantify the model uncertainty in the transverse flow predictions, which was found to depend on the incident energy and amounted to about 30% at 100 and 13% at 400 A MeV.…”

“…[12], we perform box calculations, i.e., simulations of nuclear matter enclosed in a box with imposed periodic boundary conditions. The box calculations have several advantages: (1) the initial conditions are straightforward to realize, (2) the average density remains constant in time, (3) the different aspects of heavy-ion collisions can be isolated and tested separately, e.g.…”

“…Thus, it should be noted that the codes used in this paper may not in all cases and aspects be identical to the versions of the codes compared in Ref. [12].…”

“…[12] is the departure point for the present comparison. In this context, it is of interest to gain more insight into the codes than provided by the tables in Ref.…”