Interferometry Study in High-Energy Nuclear Collisions

利用CERN-SPS的NA44聚焦谱仪，高精度测量了158GeV/核子的铅-铅碰撞产生的强子的单粒子能谱和二粒子能谱。所有的介子、介子、质子和重离子的单子谱都显示出与强子质量有关的温度参数在横向质量(Mt)中呈类Boltzmann分布。这个参数在强子质量中是线性的，从而意味着强子源以0.4倍光速的平均速度膨胀，强子冻结温度约为140 MeV。从观察到的粒子比发现，如果达到化学平衡，化学平衡温度约为170 MeV，略高于冻结温度，u夸克和d夸克的化学势约为70 MeV，而奇异夸克的化学势约为30 MeV。对两粒子能谱进行了HBT干涉研究，给出了三维HBT半径参数RL。这些参数在每核子450 GeV的pPb值和200GeV的S Pb值中总是较大的。这些HBT半径在Mt中显示出近似的平方根反比标度，与热流体模型一致。介子HBT半径的多重密度依赖关系表明，当源扩展到能量密度降到一个接近正常核的恒定值的状态时，就会发生介子冻结。

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.