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

Within the Transport Model Evaluation Project (TMEP), we present a detailed study of Sn+Sn collisions at 270A 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 Δ resonances, which have been used as probes of the nuclear symmetry energy. In this study, we prescribe a common and rather simple physical model, and follow in detail the results of 4 Boltzmann-Uehling-Uhlenbeck (BUU) models and 6 quantum molecular dynamics (QMD) models. We find that the nucleonic evolution of the collision and the nucleonic observables in these codes do not completely converge, but that 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 π^-/π^+ yield ratio, while a more effective Pauli blocking generally leads to a slightly suppressed pion yield and an enhanced π^-/π^+ yield ratio, although differences in its implementations do not have a dominating role in the differences among the codes. We specifically investigate the effect of the Coulomb force, and find that it increases the total π^-/π^+ yield ratio but reduces the ratio at high pion energies. Using 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 less than 15\%, while the uncertainty is expected to be reduced to about 4\% if the same or similar ingredients are incorporated in each code.