What Invariant One-Particle Multiplicity Distributions and Two-Particle Correlations Are Telling Us about Relativistic Heavy-Ion Collisions

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We use a nine-parameter expanding source model that includes special relativity, quantum statistics, resonance decays, and freeze-out on a realistic hypersurface in spacetime to analyze invariant pi+, K+, and K- one-particle multiplicity distributions and pi+ and pi- two-particle correlations in nearly central collisions of Pb + Pb at a laboratory bombarding momentum per nucleon of 158 GeV/c. The adjustable parameters of the model are determined by minimizing chi-square with a total of 2137 data points for the five types of data considered, which leads to an acceptable value of 1.017 for chi-square per degree of freedom. Compared to earlier results obtained with this model for nearly central collisions of Si + Au at a laboratory bombarding momentum per nucleon of 14.6 GeV/c, the present results indicate that in Pb + Pb collisions the freeze-out density is somewhat lower, the freeze-out temperature is slightly higher, the source at freeze-out is somewhat larger, and the longitudinal collective velocity is very poorly determined (because of the limited experimental coverage in rapidity). For both reactions, the freeze-out temperature is less than 100 MeV and the longitudinal and transverse collective velocities-which are anti-correlated with the temperature-are substantial. We reconcile these results with those in several previous analyses that yielded a much higher freeze-out temperature of approximately 140 MeV, and finally perform two additional studies with our expanding source model.