We extend the normal transport model to include the medium effect on pions by treating them as quasiparticles. The property of the quasipion is determined using the delta-hole model. Modeling heavy-ion collisions at intermediate energies with the new transport equations, we find that it leads to an enhanced production of pions with low kinetic energies. This gives a plausible explanation for the observed enhancement of soft pions in the Bevalac experiments. 1993 The American Physical Society.
