Comparing pion production in transport simulations of heavy-ion collisions at 270 A MeV under controlled conditions

Within the Transport Model Evaluation Project (TMEP), we present a detailed study of the performance of different transport models in Sn + Sn collisions at 270 A MeV, which are representative reactions used to study the equation of state at suprasaturation densities. We put particular emphasis on the production of pions and A resonances, which have been used as probes of the nuclear symmetry energy. In this paper, we aim to understand the differences in the results of different codes for a given physics model to estimate the uncertainties of transport model studies in the intermediate energy range. Thus, we prescribe a common and rather simple physics model, and follow in detail the results of four Boltzmann-Uehling-Uhlenbeck (BUU) models and six quantum molecular dynamics (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 pi - /pi + yield ratio, while a more effective Pauli blocking generally leads to a slightly suppressed pion yield and an enhanced pi - /pi + yield ratio. We specifically investigate the effect of the Coulomb force and find that it increases the total pi - /pi + 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 1 sigma 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 nonlinear term in the mean -field potential, are similarly used in all codes. As a result of this work, we identify the sensitive aspects of a simulation with respect to pion observables, and suggest optimal procedures in some cases. This work provides benchmark calculations of heavy -ion collisions to be complemented in the future by simulations with more realistic physics models, which include the momentum -dependence of isoscalar and isovector mean -field potentials and pion in -medium effects.