Pion multiplicity and energy spectrum in medium energy heavy-ion collisions are analyzed in a model with spherical expansion, resonance decay contribution, and chemical nonequilibrium effects. fitting the data for central La+La collision at ${\mathit{E}}_{\mathrm{lab}}$=1.35 GeV/nucleon we find that the physically reasonable freeze-out conditions require a strong collective motion to describe the pion spectrum and a positive value of pion chemical potential to fit the total pion multiplicity. The data do not lend to conclusive evidence on the presence of Δ resonance at the freeze-out.

• Received 6 July 1994

DOI:https://doi.org/10.1103/PhysRevC.51.1465