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

TL;DR

This study compares various transport models in simulating pion production in Sn+Sn collisions at 270A MeV, highlighting differences due to model types, physics assumptions, and implementation details, and demonstrating convergence under standardized conditions.

Contribution

It provides a detailed comparison of BUU and QMD transport models under controlled conditions, identifying sources of discrepancies and demonstrating improved convergence with standardized strategies.

Findings

Higher maximum density increases pion yield and reduces $rac{ ext{π}^-}{ ext{π}^+}$ ratio.

More effective Pauli blocking slightly suppresses pion yield and increases the $rac{ ext{π}^-}{ ext{π}^+}$ ratio.

Coulomb force affects the total and high-energy pion yield ratios.

Abstract

Within the TMEP, we present a detailed study of the performance of different transport models in Sn+Sn collisions at $270A$ MeV, and put particular emphasis on the production of pions and $\Delta$ resonances, which have been used as probes of the nuclear symmetry energy. We prescribe a common and rather simple physics model, and follow in detail the results of 4 BUU models and 6 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 non-linear term in the mean-field potential, are similarly used in all codes.