Thermal and chemical freeze-out in heavy ion collisions

The NA44 experiment has measured normalized single particle distributions for charged pions, kaons and protons as functions of transverse mass near mid-rapidity in 158 A GeV/c Pb+Pb collisions. From the particle mass dependence for the observed mT distributions, we are able to deduce a value of about 120 MeV for the temperature at the thermal freeze-out. On the other hand, from the observed ratios of the rapidity densities, we are able to determine values of the chemical potentials for light and strange quarks and the chemical freeze-out temperature of approximately 140 MeV. These values seem to justify a scenario in P+A and A+A collisions at the CERN-SPS energies in which a particle source of very high energy density is produced. It then cools due to fast expansion. During the expansion, the fireball first reaches a chemical freeze-out and later reaches thermal freeze-out. (author)