Systematic study of the pi(-)/pi(+) ratio in heavy-ion collisions with the same neutron/proton ratio but different masses

A systematic study of the pi(-)/pi(+) ratio in heavy-ion collisions with the same neutron/proton ratio but different masses can help single out effects of the nuclear mean field on pion production. Based on simulations using the IBUU04 transport model, it is found that the pi(-)/pi(+) ratio in head-on collisions of Ca-48 + Ca-48, Sn-124 + Sn-124, and Au-197 + Au-197 at beam energies from 0.25 to 0.6 GeV/nucleon increases with increasing the system size or decreasing the beam energies. A comprehensive analysis of the dynamical isospin fractionation and the pi(-)/pi(+) ratio as well as their time evolution and spatial distributions demonstrates clearly that the pi(-)/pi(+) ratio is an effective probe of the high-density behavior of the nuclear symmetry energy.

National Natural Science Foundation of China 10675148 10635080 10575071 10675082 MOE of China NCET-05-0392 Shanghai Rising-Star Program 06QA14024 SEM of China National Basic Research Program of China 2007CB815004 US National Science Foundation PHY-0652548 PHY-0757839 Research Corporation 7123 Texas Coordinating Board of Higher Education 0035650004- 2007