Symmetry energy and pion production in the Boltzmann-Langevin approach

Based on the improved isospin-dependent Boltzmann-Langevin model which incorporates the dynamical fluctuations, we study the π production in central heavy ion collisions at different incident energies from 250 to 1200 AMeV. It is found that the π multiplicity is sensitive to the nuclear equation of state. At π subthreshold energy, the fluctuations have a larger effect on the π multiplicity. The π^- /π⁺ ratios as a probe of nuclear symmetry energy are calculated with different stiffness of symmetry energy. The results favor a supersoft symmetry energy of the potential term in comparison with the FOPI data, which supports the one obtained by the usual Boltzmann-Uehling-Uhlenbeck model.