$\Sigma^-/\Sigma^+$ ratio as a candidate for probing the density   dependence of the symmetry potential at high nuclear densities
  Based on the UrQMD model, we have investigated the influence of the symmetry potential on the negatively and positively charged $\pi$ and $\Sigma$ hyperon production ratios in heavy ion collisions at the SIS energies. We find that, in addition to $\pi^-/\pi^+$ ratio, the $\Sigma^-/\Sigma^+$ ratio can be taken as a sensitive probe for investigating the density dependence of the symmetry potential of nuclear matter at high densities (1-4 times of normal baryon density). This sensitivity of the symmetry potential to both the $\pi^-/\pi^+$ and $\Sigma^-/\Sigma^+$ ratios is found to depend strongly on the incident beam energy. Furthermore, the $\Sigma^-/\Sigma^+$ ratio is shown to carry the information about the isospin-dependent part of the $\Sigma$ hyperon single-particle potential. 
Qingfeng LI, Zhuxia LI, Enguang Zhao, and Raj K. Gupta