There are two type of scales present simultaneously in the space-like as well as in the time-like directions in a model-class describing a cylindrically symmetric, finite, three-dimensionally expanding boson source. One type of the scales is related to the finite lifetime or geometrical size of the system, the other type is governed by the rate of change of the local momentum distribution in the considered temporal or spatial direction. The parameters of the Bose-Einstein correlation function may obey an M t -scaling, as observed in S + P b and P b + P b reactions at CERN SPS. This M t -scaling may imply that the Bose-Einstein correlation functions view only a small part of a big and expanding system. The full sizes of the expanding system at the last interaction are shown to be measurable with the help the invariant momentum distribution of the emitted particles. A vanishing duration parameter can also be generated, with a specific M t dependence, in the considered model-class.

We discuss Bose-Einstein correlation functions and momentum distributions which describe boson-emitting systems containing a central part surrounded The results are applied to high energy heavy ion data taken at CERN SPS.New insights are obtained for the description of low transverse momentum enhancement of pions. Z. Phys. C in press *

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200A GeV S 1 S and 158A GeV Pb 1 Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions. The purpose of studying ultrarelativistic heavy-ion collisions is to understand the nature of hadronic matter under extreme conditions. Specifically, we are interested in a new form of matter, quark-gluon plasma, which may be produced in such collisions. Transverse momentum distributions are one of the most common tools used in studying high energy collisions. This is because the transverse motion is generated during the collision and hence is sensitive to the dynamics. More than 45 years ago, Fermi proposed a statistical method [1] to understand the results of high energy hadron-hadron collisions. Because of saturation of the phase space, the multiparticle production resulting from the high energy elementary collisions is consistent with a thermal description [1][2][3]. In heavy-ion collisions hydrodynamical behavior, that is, local thermal equilibrium and collective motion, may be expected because of the large number of secondary scatterings.It is now possible to identify and quantitatively measure the collective motion by systematic studies of results from different collision systems, using light (Si at BNL and S at CERN) and heavy (Au at BNL and Pb at CERN) ion beams [4][5][6]. A high degree of nuclear stopping and a strong Coulomb effect (also due to the high stopping) have already been reported in Pb 1 Pb central collisions [7,8]. In this Letter, we present transverse momentum distributions of pions, kaons, and protons, measured in the NA44 spectrometer, from Pb 1 Pb and S 1 S collisions. Results of calculations from a hydrodynamical model [5] will be used to aid in this analysis.The NA44 magnetic focusing spectrometer consists of two room-temperature dipoles and three superconducting quadruples. Particles originating from the target are focused at a plane about ten meters downstream and detected by a tracking system consisting of a pad chamberstrip chamber-scintillator hodoscope complex. Particle identification is done with two threshold Cherenkov counters and two highly segmented TOF hodoscopes. The phase-space coverage (transverse momentum p T vs rapidity y) is determined by the combination of the spectrometer angle (relative to the beam direction) and the nominal momentum setting of the magnets. The momentum resolution is typically s p ͞p # 0.2% and the TOF counters have an average time resolution of 100 ps. More details of the spectrometer can be found elsewhere [9].The spectrometer momentum range is 620% around the nominal values of 4 and 8 GeV͞c. For kaons and protons, the 8 GeV͞c setting was used and the rapidity coverage is (2.5-3.4) and (2.4-2.8) for kaons and protons, respectively. Two angular settings (44 and 130 mrad) were utilized in order ...