Using a new technique to solve the kinetic equation including Bose statistics, we show that the shape of the pion transverse momentum spectrum is sensitive to the hadronization time in ultrarelativistic heavy ion collisions. For a pure pion gas, the magnitude of the soft pion component observed in central 200 GeV/nucleon O+Au collisions is reproduced by an effective pionization time of about 7 fm/c. This explanation implies that the hadrons are produced out of chemical equilibrium.

• Received 6 May 1991

DOI:https://doi.org/10.1103/PhysRevC.45.1403