Modeling Relativistic Heavy-Ion Collisions

The purpose of Transport Theory

A promising approach to connect the transient QGP state with the experimentally observable hadronic final state is the application of transport theory. Transport theory offers the possibility to cast the entire time evolution of the heavy-ion reaction - from its initial state to freeze-out - into one consistent framework. In microscopic transport models the full space-time evolution of all microscopic degrees of freedom - either all hadrons present in the system or alternatively (at higher beam energies) partons - is calculated from the initial state to the final freeze-out, which allows for quantitative predictions of QGP signatures.

Cartoon of a Ultra-relativistic heavy-ion collision. Left to right: the two nuclei approach, collide, form a QGP, the QGP expands and hadronizes, finally hadrons rescatter and freeze out

Microscopic Transport Theory

Microscopic Transport Theory treats the microscopic substructure of the colliding nuclei explicitly, i.e. the trajectories and interactions of all protons, neutrons and newly created baryons and mesons in the case of hadronic transport models or quarks and gluons in the case of a Parton Cascade are followed individually. Microscopic transport models are most useful to tackle questions concerning particle production, fluctuations and equlilibration mechanisms - they do not rely on any mean-field or equilibrium assumptions.

Ultra-Relativistic Quantum Molecular Dynamics (UrQMD)

The Ultra-relativistic Quantum-Molecular-Dynamics (UrQMD) model is currently among the most advanced microscopic string/hadron transport model available. It has been successfully applied to heavy-ion reactions at the Bevalac, SIS, AGS and SPS accelerator facilities, spanning incident beam energies from 0.5 GeV per nucleon (at the Bevalac and SIS facilities) up to 200 GeV per nucleon (at the SPS facility). Main goals in the application of the UrQMD model are to gain an understanding about the following physical phenomena within a single transport model:

  • Creation of dense hadronic matter at high temperatures
  • Properties of nuclear matter, Delta & Resonance matter
  • Creation of mesonic matter and of anti-matter
  • Creation and transport of rare particles in hadronic matter.
  • Creation, modification and destruction of strangeness in matter
  • Emission of electromagnetic probes

The UrQMD model has also been used as a component of various hybrid transport approaches, e.g.

  • Hydro+UrQMD
  • UrQMD+deexcitation models for nuclear waste calculations
  • GEANT4
  • CORSIKA+UrQMD for air-shower simulations

Click here for a UrQMD movie of an Pb+Pb collision at CERN/SPS energies (mpg format).

Parton Cascade Model (PCM)

The Parton Cascade Model (PCM) is a microscopic approach with quarks and gluons as degrees of freedom and interactions based on perturbative QCD. The PCM is ideally suited to study the physics of deconfined colored quanta at the RHIC and LHC accelerators, particularly in the large momentum domain. It allows for the description of the dynamics of the pre-equilibrium early, hot and dense deconfined reaction phase and connects it to experimentally observable quantities. Key physics issues to be addressed in this approach are the mechanisms of parton equilibration, the microscopic dynamics of jet-energy loss in a deconfined medium, the production of heavy quarks and the emission of photons and dileptons in the early, non-equilibrium phase of the reaction. Currently our group is active in the development of the VNI/BMS implementation of the Parton Cascade Model.

Hybrid Macro+Micro transport approaches

Hybrid macroscopical/microscopical transport models, employing hydrodynamics for the early dense reaction phase and microscopic non-equilibrium dynamics for the later, dilute reaction stages offer an alternative approach and are particularly interesting for the investigation of the QGP equation of state in the energy domains beyond beam energies of a 100 GeV per nucleon, covered by the SPS, RHIC and LHC accelerators.