Effects of Pauli blocking on pion production in central collisions of neutron-rich nuclei

TL;DR

This paper improves the treatment of Pauli blocking in heavy-ion collision simulations, leading to more accurate predictions of pion ratios that are crucial for understanding the symmetry energy at high densities.

Contribution

An improved method for Pauli blocking in AMD+JAM simulations using the Wigner function, enhancing the accuracy of pion production predictions in neutron-rich heavy-ion collisions.

Findings

Stronger Pauli blocking observed with the new method.

Higher $ ext{π}^-/ ext{π}^+$ ratio when using the improved blocking.

Effect comparable to symmetry energy sensitivity.

Abstract

Pauli blocking is carefully investigated for the processes of $NN \rightarrow N \Delta$ and $\Delta \rightarrow N \pi$ in heavy-ion collisions, aiming at a more precise prediction of the $\pi^-/ \pi^+$ ratio which is an important observable to constrain the high-density symmetry energy. We use the AMD+JAM approach, which combines the antisymmetrized molecular dynamics for the time evolution of nucleons and the JAM model to treat processes for $\Delta$ resonances and pions. As is known in general transport-code simulations, it is difficult to treat Pauli blocking very precisely due to unphysical fluctuations and additional smearing of the phase-space distribution function, when Pauli blocking is treated in the standard method of JAM. We propose an improved method in AMD+JAM to use the Wigner function precisely calculated in AMD as the blocking probability. Different Pauli blocking methods are compared in heavy-ion collisions of neutron-rich nuclei, ${}^{132}\mathrm{Sn}+{}^{124}\mathrm{Sn}$, at 270 MeV/nucleon. With the more accurate method, we find that Pauli blocking is stronger, in particular for the neutron in the final state in $NN \rightarrow N \Delta$ and $ \Delta \to N\pi$, compared to the case with a proton in the final state. Consequently, the $\pi^-/\pi^+$ ratio becomes higher when the Pauli blocking is improved, the effect of which is found to be comparable to the sensitivity to the high-density symmetry energy.