Microscopic Models for Ultrarelativistic Heavy Ion Collisions

TL;DR

The paper surveys microscopic transport theory for heavy ion collisions and presents UrQMD as a comprehensive, energy-spanning transport framework. It details the model’s initialization, covariant/relativistic dynamics, an extensive collision term with resonances and string fragmentation, and a broad array of observables from stopping and spectra to dileptons and flow. It demonstrates UrQMD’s ability to reproduce many hadrochemical and dynamical features across SIS to SPS energies while highlighting tensions in resonance handling, antimatter dynamics, and potential signals of deconfinement. The work emphasizes that non-equilibrium, hadronic dynamics with string degrees of freedom can explain many bulk observables, while also outlining future directions toward relativistic many-body forces, off-shell effects, and partonic degrees of freedom for collider energies.

Abstract

In this paper, the concepts of microscopic transport theory are introduced and the features and shortcomings of the most commonly used ansatzes are discussed. In particular, the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model is described in great detail. Based on the same principles as QMD and RQMD, it incorporates a vastly extended collision term with full baryon-antibaryon symmetry, 55 baryon and 32 meson species. Isospin is explicitly treated for all hadrons. The range of applicability stretches from $E_{lab}< 100$ MeV/nucleon up to $E_{lab}> 200$ GeV/nucleon, allowing for a consistent calculation of excitation functions from the intermediate energy domain up to ultrarelativistic energies. The main physics topics under discussion are stopping, particle production and collective flow.

Figures (86)

• Figure 1: Phase diagram of hadronic matter: Only the point at ground state density is well known; the dashed lines show areas probed in the course of heavy ion reactions.
• Figure 2: Feynman diagrams for the T-matrix approximation
• Figure 3: Feynman diagrams for the Hartree-Fock approximation
• Figure 4: Feynman diagrams for the Born approximation
• Figure 5: Feynman diagrams for the self-energy in the Born approximation