  We present the simulations of heavy-ion collisions in terms of
participant-spectator matter. We find that this matter depends crucially on the
collision dynamics and history of the nucleons. The important changes in the
momentum space are due to the binary nucleon-nucleon collisions experienced
during the high dense phase. This was otherwise not possible with mean field
alone. The collisions push the colliding nucleons into midrapidity region
responsible for the formation of participant matter. This ultimately leads to
thermalization in heavy-ion collisions.
