Dynamics of pions in ultrarelativistic heavy ion collisions

In an ultrarelativistic collision of heavy nuclei, such as 197Au, at an energy Vs above roughly 10 GeV per nucleon pair, nuclear transparency effects are expected to give rise to a baryon-poor central region. Matter in the central region can begin either as a pion fluid or as a quark-gluon plasma, depending on the initial energy density. A pion fluid is also formed when the expanding plasma hadronizes. In this thesis, we study the dynamics of pions in the post-hadronization central region to develop probes of quark-gluon plasma formation. In parallel, we consider the evolution of the pion fluid formed directly as the nuclei first interpenetrate, both as a benchmark problem for comparison to plasma evolution, and because collisions at a fixed beam energy can generate a range of initial energy densities through fluctuations. To begin, we use kinetic theory to study the pion fluid in the hydrodynamic, nearlocal-equilibrium regime, in which interactions result in dissipative phenomena such as viscosity and thermal conduction. We derive the first and second viscosities and thermal conductivity of pion matter in terms of relaxation times. We then calculate the relaxation times, and estimate the importance of dissipation in the expansion, assuming that pion number is effectively conserved. We calculate the rates of pion-number-changing reactions, and show that this assumption is valid for the conditions expected in the central region. We investigate the thermalization and freezeout stages of the nuclear collision, in which the system is far from local equilibrium. The rapid expansion of a directly-formed (as opposed to post-hadronization) pion fluid results in a novel, partially thermalized flow, in which the degree of thermalization is measured by the observed pion multiplicity. We develop a model of the freezeout of the conserved pion fluid, which we use to construct the invariant pion distribution in terms of the calculated phase-space distribution function. These results imply that the partial thermalization of the pion gas results in final-state correlations that may forge the signature of quark-gluon plasma formation.U of I OnlyThesi