EOS from exapanding and clusterizing matter in heavy-ion collisions*

In heavy-ion collisions at several hundred MeV/nucleon, a compressed nuclear system is formed up to about twice the saturation density and then rapidly expands. It has been a theoretical challenge to extract information on nuclear matter properties such as the EOS of isospin-asymmetric nuclear matter. Even though one may be tempted to regard the global collision evolution as one-body dynamics in phase space, light clusters and heavier fragment nuclei are copiously produced and measured in experiments. The many-body correlations to form them can be so strong that they affect the global collision evolution.

Transport Model Evaluation Project is advancing a better understanding of standard transport models, while some extended models have individual features. The antisymmetrized molecular dynamics (AMD) model incorporates the cluster formation process in the collision term, which allows us to describe the copious formation of clusters. A recent AMD calculation compared with the SpiRIT experimental data for collisions of Sn isotopes indicates that clusters such as tritons carry information on the symmetry energy in the compressed and expanding system. However, future theoretical efforts are needed to fully describe the cluster observables.

Pions have been expected to be a good probe for high-density EOS. In the SpiRIT systems at 270 MeV/nucleon, the pion multiplicity is sufficiently low so the pion observables can be predicted by using the nucleon information from AMD in another transport model, such as the JAM code. Recently, we developed a new code that we call sJAM, extending the collision term in JAM by treating momentum-dependent potentials consistently. This study allows a detailed understanding of how the pion observables such as the charged pion ratio is generated reflecting nuclear matter properties such as the high-density symmetry energy. The current conclusion of our AMD+sJAM study is that the charged pion ratio is most strongly sensitive to the momentum dependence of symmetry potential.