Effects of pion potential and nuclear symmetry energy on the   $\pi^{-}/\pi^{+}$ ratio in heavy-ion collisions at beam energies around the   pion production threshold
  Within the framework of the isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU) transport model, we studied the effects of the pion potential and the symmetry energy on the pion production in the central $^{197}Au+^{197}Au$ collisions around the pion production threshold. It is found that the pion potential has opposite effects on the value of $\pi^-/\pi^+$ ratio at low and high pion energies. The effect of the pion potential on the total $\pi^-/\pi^+$ ratio becomes larger in heavy-ion collisions at beam energies below the pion production threshold. And at beam energies below the pion production threshold, with the pion potential, the effect of the symmetry energy on the $\pi^-/\pi^+$ ratio becomes smaller compared with that above the pion production threshold. 
Wen-Mei Guo, Gao-Chan Yong, Hang Liu, Wei Zuo
nucl-th nucl-ex