  Based on the isospin dependent transport model IBUU, the pion production and
its absorption are thoroughly studied in the central collision of $Au+Au$ at a
beam energy of 400 MeV/nucleon. It is found that the pions are firstly produced
by the hard $\Delta$ decay at the average density around $1.75\rho_{0}$,
whereas about 18\% of them are absorbed absolutely in the subsequent inelastic
collisions. For the free pions observed, more than half of them have been
scattered for one or more times before they are free from matter. And the more
scattering numbers of pions, the higher the momentum they possess. These pions,
due to longer time of their existence in high density nuclear matter, carry
more information about the symmetry energy of the nuclear matter at high
densities.
