Pion and Proton "Temperatures" in Relativistic Heavy-Ion Reactions

Pion and proton production are measured to investigate thermal equilibrium in central collisions of ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$+KCl at 1.8 GeV/nucleon. The bulk of the pion yield is isotropic in the c.m. system, with an apparent temperature of 58±3 MeV, much lower than the 118±2 MeV of the protons. It is shown that the low pion "temperature" can be explained by the decay kinematics of delta resonances in thermal equilibrium. A (5±1)% component in the pion spectrum is, however, found to have a temperature of 110±10 MeV. The effect on the spectra of possible contributions from collective radial flow is discussed.