Probing nuclear symmetry energy with the sub-threshold pion production

Within the framework of the semiclassical Boltzmann-Uehling-Uhlenbeck transport model, we investigate the effects of symmetry energy on the sub-threshold pion using the isospin MDI interaction with the stiff and soft symmetry energies in the central collision of ⁴⁸Ca + ⁴⁸Ca at the incident beam energies of 100, 150, 200, 250 and 300 MeV/nucleon. We find that the ratio of π^-/π⁺ of sub-threshold charged pion production is greatly sensitive to the symmetry energy, particularly around 100 MeV/nucleon energies. Large sensitivity of sub-threshold charged pion production to nuclear symmetry energy may reduce uncertainties of probing nuclear symmetry energy via heavy-ion collision. (authors)