Interferometry Study in High-Energy Nuclear Collisions

利用欧洲核子研究中心（CERN-SPS）的NA44聚焦光谱仪，高精度测量了158 GeV /核子下Pb+Pb碰撞产生的强子的单粒子和双粒子光谱。π子、介子、质子和氘核的单谱在横质量（mT）中均表现出类似玻尔兹曼的分布，且温度参数与强子质量有关。该参数在强子质量中是线性的，这意味着强子源膨胀的平均速度为0.4倍光速，强子冻结温度约为140 MeV。从观察到的粒子比中发现，如果达到化学平衡，则化学平衡温度约为170 MeV，略高于冻结温度，u和d夸克的化学势约为70 MeV，而奇夸克的化学势几乎等于30 MeV。对两粒子光谱进行了HBT干涉测量研究，给出了三维HBT半径参数RL、RTO和RTS。在450gev时，Pb+Pb的这些参数值总是大于p+Pb和S+Pb在200gev时的相应值。这些HBT半径在mT中显示出近似的反平方根尺度，与热流体动力学模型一致。π介子HBT半径的多重密度依赖性表明，当源膨胀到能量密度下降到接近正常原子核的恒定值时，π介子发生冻结。

By means of the NA44 focussing spectrometer at the CERN-SPS,we have carried out a high-precision measurement of one-and two-particie spectra of hadrons emitted from Pb+Pb collisions at 158 GeV per nucleon. AII of singles spectra for pions, kaons, protons and deuterons show a Boltzmann-like distribution in the transverse mass (mT) with hadron-mass dependent temperature parameter. The parameter is linear in hadron mass, thereby implying that the hadron source expands with an average velocity of 0.4 times the velocity of light and the hadron-freezeout temperature is about 140 MeV.From the observed particle ratios it is found that if a chemical equilibrium is attained, the chemical equilibrium temperature is about 170 MeV,being somewhat higher than the freezeout temperature, and the chemical potential for u and d quarks is about 70 MeV,whereas that for strange quark is nearly equal to 30 MeV.HBT interferometry study has been carried out for two-particle spectra, giving three dimentional HBT radius parameters RL,RTO and RTS.Values of these parameters for the Pb+Pb are always larger than the corresponding ones in p+Pb at 450 GeV and S+Pb at 200 GeV per nucleon. These HBT radii show an approximate inverse-square-root scaling in mT, consistently with thermal hydrodynamical model. The multiplicity density dependence of the HBT radii for pions shows that the pion freezeout occurs when the source expands up to a state in which the energy density goes down to a constant value which is close to that of normal nucleus.