Study of lead-lead collisions at 160 GeV per nucleon

1.我们在欧洲核子研究中心SPS上进行了高能重离子实验NA44。对160 GeV /核子的铅-铅碰撞和200 GeV /核子的硫-铅碰撞的实验数据进行了分析。从单粒子光谱来看，在这些碰撞中产生的强子源的温度与强子质量有关。然而，我们的分析表明，这种质量依赖是由于源的流体动力学膨胀，真实温度与强子种类无关。利用双粒子干涉（HBT效应），得到了三维分析中的源尺寸参数（HBT半径）。任何测量的HBT半径都与横向质量m_T.4的平方根的倒数成正比。这种依赖关系与水动力模型的预测相一致，似乎表明横向流动的存在。介子和介子源大小的明显差异归因于它们的m_T.5平均值的差异。从横向HBT半径，我们在世界上第一次能够确定硫+铅碰撞的冻结时间分布宽度为2 ~ 3.5 fm/c，这似乎比QGP相变的预期要小得多。HBT半径与多重密度的立方根成正比，表明介子冻结发生在源粒子密度降至某一恒定值的条件下。

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.