Possible explanation on the low pT shape of the transverse pion distribution in ultrarelativistic nuclear collision

The transverse momentum distribution of pions in ultrarelativistic nuclear collision is calculated by the Bjorken scaling hydrodynamic model. Satisfactory agreement could be obtained with the experimental data which shows a concave shape in the low pT region if three temperatures are used: 200 MeV for the fluid expansion time τ 2 fm/c, 140 MeV for 2 fm/c > τ < 3 fm/c and 80 MeV for τ > 3 fm/c. If is suggested that the observed pion temperature should be a variable in a small temperature interval during the fluid expansion