Pion and proton ''temperatures'' in relativistic heavy-ion reactions

Pion and proton production are measured to investigate thermal equilibrium in central collisions of 40Ar+KCl at 1.8 GeV/nucleon. The bulk of the pion yield is found to be 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 kinematics of the decay of delta resonances in thermal equilibrium. A (5 +- 1)% isotropic component in the pion spectrum is, however, found to have a temperature of 110 +- 10 MeV, near that of the protons and Δ's. The effect on the spectra of possible contributions from collective radial is discussed