Universal pion freeze-out phase-space density Original Research Article

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 ƒ = [exp(E/T) − 1]−1 at T ∼ 120 MeV, modified for radial expansion. π-p data are also consistent with ƒ, but at T ∼ 180 MeV and without radial flow.