Probing the Dynamics of Ultrarelativistic Nucleus - Collisions.

The stopping and scaling hydrodynamical regimes for relativistic nucleus-nucleus collisions are presented and contrasted. Longitudinal boost invariant transverse hydrodynamics with a first order phase transition is shown to lead to pure quark-gluon plasma lifetimes of the order of 10 fm/c and mixed phases of the order of 50 fm/c for U + U ultrarelativistic collisions. Direct comparisons between theory and experiment are made for nucleus-nucleus collisions (cosmic ray data) and also for high-multiplicity fluctuations of pp collisions, by calculating the correlation between the mean transverse momentum and the multiplicity per unit rapidity. Pseudorapidity distributions are shown to be approximately Gaussian in shape at AGS and CERN energies. Quantitatively, the widths are consistent with Landau's hydrodynamical model and inconsistent with a spherical fireball model and with Bjorken's hydrodynamical scaling model. There are no statistically significant correlations in azimuthal angle of the observed charged secondaries. Analysis of the two-particle pseudorapidity correlation functions and of the pseudorapidity gap distributions provide no evidence of significant cluster production. The data is consistent with a Monte Carlo simulation based on independent emission of the secondary particles. Using the method of scaled factorial moments an intermittent behavior of the fluctuations was observed in both proton and oxygen beam interactions with AgBr at 200 GeV/n. The p_{rm T} distributions are calculated using the longitudinal boost invariant transverse hydrodynamics and compared to S + S collisions at 200 GeV/n. The medium and high p _{rm T} parts of the distribution can be reproduced within the context of a collective flow arising from a mixed phase, whereas the low p_ {rm T} component of the distribution cannot be explained by this hypothesis.