The researchers of Nuclear Theory Group at the Institute of Modern Physics, Chinese Academy of Sciences (IMP) have investigated effects of in-medium NN elastic scattering cross section and nuclear symmetry energy on the sub-threshold pion production and obtained some interesting results. 

The researchers studied effects of in-medium NN cross section on the charged pion ratio. As is shown in Fig.1, the effects of the in-medium NN elastic cross section on the value of the pion-/pion+ ratio become larger and larger with decreasing beam energy. At the lower beam energy of 100 MeV/nucleon, the effects of in-medium NN elastic cross section on this ratio can reach about 40%. 

In order to reduce the effects of in-medium and retain effects of the symmetry energy on the charged pion ratio, the double ratio of the charged pion ratio is studiedby using two reaction systems of the same neutron-rich and neutron-poor isotopes.It is found that even using the double ratio of observables, the effects of the in-medium NN cross section are still very obvious. While the double ratio of the charged pion ratio from two reaction systems (with the same mass number of system but different isotopes) 132Sn + 124Sn and 128Pm + 128Pm (as shown in Fig.2) keeps the effects of the symmetry energy, the effects of the in-medium NN elastic scattering cross section are almost fully canceled out. The reason is that the latter-two neutron-rich and neutron-poor reaction systems have the same baryon number evolutions, and distributions of baryon density in the two reactions are almost the same. 

The related articles have been published inPHYSICAL REVIEW C90, 044605 (2014). http://journals.aps.org/prc/abstract/10.1103/PhysRevC.90.044605 

Fig.1 Effects of in-medium NN cross section on the charged pion ratio (Image by IMP). 

Fig.2 Effects of medium and symmetry energy on the double charged pion ratio from 132Sn + 124Sn and 128Pm + 128Pm at the beam energy of 100 MeV/nucleon (Image by IMP).