Probing nuclear symmetry energy at high densities using pion, kaon, eta and photon productions in heavy-ion collisions

The high-density behavior of nuclear symmetry energy is among the most uncertain properties of dense neutron-rich matter. Its accurate determination has significant ramifications in understanding not only the reaction dynamics of heavy-ion reactions, especially those induced by radioactive beams, but also many interesting phenomena in astrophysics, such as the explosion mechanism of supernova and the properties of neutron stars. The heavy-ion physics community has devoted much effort during the last few years to constrain the high-density symmetry using various probes. In particular, the \( \pi^{-}\)/\( \pi^{+}\) ratio has been most extensively studied both theoretically and experimentally. All models have consistently predicted qualitatively that the \( \pi^{-}\)/\( \pi^{+}\) ratio is a sensitive probe of the high-density symmetry energy especially with beam energies near the pion production threshold. However, the predicted values of the \( \pi^{-}\)/\( \pi^{+}\) ratio are still quite model dependent mostly because of the complexity of modeling pion production and reabsorption dynamics in heavy-ion collisions, leading to currently still controversial conclusions regarding the high-density behavior of nuclear symmetry energy from comparing various model calculations with available experimental data. As more \( \pi^{-}\)/\( \pi^{+}\) data become available and a deeper understanding about the pion dynamics in heavy-ion reactions is obtained, more penetrating probes, such as the K +/K 0 ratio, \( \eta\) meson and high-energy photons are also being investigated or planned at several facilities. Here, we review some of our recent contributions to the community effort of constraining the high-density behavior of nuclear symmetry energy in heavy-ion collisions. In addition, the status of some worldwide experiments for studying the high-density symmetry energy, including the HIRFL-CSR external target experiment (CEE) are briefly introduced.