Universal pion freeze-out phase-space density

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