Universal pion freeze-out phase-space density

Abstract

Results on the pion phase-space density at freeze-out in sulphur-nucleus, Pb-Pb and pi-p collisions at the CERN SPS are presented. All heavy-ion reactions are consistent with the thermal Bose-Einstein distribution f=1/[exp(E/T)-1] at T ~ 120 MeV, modified for radial expansion. pi-p data are also consistent with f, but at T ~ 180 MeV and without radial flow.