Theory of relativistic heavy ion collisions

Abstract

The current status of heavy ion physics from medium energies to the quark gluon plasma is discussed in the light of several theoretical approaches. Relativistic mean field theory is discussed. The nuclear equation of state at high density and temperature is investigated in a nuclear fluid dynamic model: nuclear matter is considered as a relativistic interacting Bose and Fermi gas of π and η mesons, photons, and nucleonic resonances. At the microscopic level, the approach to local kinetic equilibrium in relativistic heavy ion collisions is studied by following the time evolution of the Wigner function in configuration and momentum space using the Vlasov-Uehling-Uhlenbeck theory. This theoretical approach includes the nuclear mean field, two body collisions, particle production, relativistic kinematics, and the Pauli principle. A Newtonian Force Model, TDHF, the Vlasov equation, the IntraNuclear Cascade model, macroscopic Nuclear Fluid Dynamics, and a simple shock model are studied as reference cases. In the VUU theory, rapid equilibration of the participant region is observed within time spans on the order of 10 fm/c. Total stopping of the projectile occurs at small impact parameters: a sidesplash of nuclear matter is predicted due to the interplay of the nuclear compressional energy and collisions. These theoretical approaches are compared to the experimental data. The pion yields, single particle spectra, and kinetic energy flow angular distributions are found to be sensitive to the nuclear compressibility and the Pauli principle: preliminary evidence for a surprisingly stiff nuclear equation of state is presented. Nuclear fragmentation or complex particle production is studied in a quantum statistical model that includes the isotopes up to Ne and also by applying a six dimensional coalescence model to the VUU final state.