Effects of nuclear symmetry energy and in-medium NN cross section in heavy-ion collisions at beam energies below the pion production threshold

Based on the isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU04) transport model, we explored effects of in-medium $NN$ elastic scattering cross section and nuclear symmetry energy on the sub-threshold pion production in $^{132}Sn+^{124}Sn$ reaction. We find that with decrease of incident beam energy, effects of in-medium $NN$ elastic scattering cross section on the $\pi^{-}/\pi^{+}$ ratio are larger than that of the symmetry energy although the latter may be also larger. While keeping the effect of symmetry energy, the double ratio of $\pi^{-}/\pi^{+}$ from neutron-rich and neutron-poor reaction systems (with the same mass number of system) $^{132}Sn+^{124}Sn$ and $^{128}Pm+^{128}Pm$ almost fully cancels out the effects of in-medium $NN$ elastic scattering cross section.