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

Abstract

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\ifmmode\pm\else\textpm\fi{}3 MeV, much lower than the 118\ifmmode\pm\else\textpm\fi{}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\ifmmode\pm\else\textpm\fi{}1)% component in the pion spectrum is, however, found to have a temperature of 110\ifmmode\pm\else\textpm\fi{}10 MeV. The effect on the spectra of possible contributions from collective radial flow is discussed.