Study of lead-lead collisions at 160 GeV per nucleon

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1.We have carried out high-energy heavy-ion experiment NA44 at the CERN SPS.The experimental data on lead-lead collisions at 160 GeV per nucleon as well as those on sulphur-lead collisions at 200 GeV per nucleon have been analyzed.2.From single particle spectra, the temperature of the hadron source created in these collisions is found to depend on the hadron mass. However, our analysis shows that this mass dependence is due to hydrodynamical expansion of the source and the true temperature is independent of hadron species.3.From two-particle interferometry (HBT effect) , we have obtained source size parameters (HBT radii) in three-dimensional analysis. Any measured HBT radii are proportional to the inverse of the square-root of transverse mass m_T.4.This dependence is consistent with the predictions of hydrodynamical model and seems to show the existence of trnasverse flow. The apparent difference of the source size between pions and kaons is ascribed to the difference between the average values of their m_T.5.From transverse HBT radii, for the first time in the world, we have been able to determine the width of freeze-out time distribution for sulphur+ lead collisions to be 2 to 3.5 fm/c, which seems substantially smaller than that expected from QGP phase transition.6.The HBT radii are proportional to the cube-root of the multiplicity density, thereby suggesting that the pion freeze-out takes place under the condition that particle density of the source goes down to a certain constant value.