NSCL, Michigan State University East Lansing, UNITED STATES
Ratio of near-threshold charged-pion yields has been proposed as a sensitive probe of supranormal symmetry-energy. Since then, conflicting conclusions have been reached by comparing transport-model predictions of the pion yields to FOPI data, alternatively claiming soft and stiff supranormal symmetry energy. In the models, the near-threshold pion production proceeds through production and decay of delta resonances. Ambiguities in the processes involve the delta-resonance lifetime and the mean fields felt by nucleons, pions and resonances. Within the pBUU transport model, we find a rather strong sensitivity of the net near-threshold pion yields to the momentum dependence of nucleonic mean fields. We find it difficult to find a simultaneous description of the measured pion yields and nucleon collective flows. Unlike elsewhere, we find practically no sensitivity of net charged pion yields to the symmetry energy. However, we find a strong sensitivity to the symmetry energy in the charged pion ratio as a function of energy of emitted particles, with the high-energy tails being especially sensitive. The latter finding is consistent with the energetic pions being emitted from high-density matter.