Microscopic Reaction Dynamics at SPS and RHIC

Transport Theory offers the unique capability of connecting experimentally observable quantities in a relativistic heavy ion collision with its time evolution and reaction dynamics , thus giving crucial insights into the possible formation of a transient deconfined phase of hot and dense matter, the Quark-Gluon-Plasma (for reviews on QGP signatures, see e.g. [1,2]). Figure 1 provides an overview of different transport theoretical ansatzes currently on the market for the description of a relativistic heavy ion collision at RHIC energies. The timeline shows a best case scenario for what to expect: the formation of a QGP with subsequent hadronization and freeze-out. Bands with solid lines denote the safe range of applicability for the respective transport approach, whereas dashed/dotted bands refer to areas in which the approach is still applied but where the assumptions on which the approach is based upon may be questionable or not valid anymore. The initial state and early pre-equilibrium phase are best described in Classical Yang-Mills theory (CYM) [3] or Lattice Gauge Transport (LGT) [4] calculations-only these classes of models treat the coherence of the initial state correctly, but do not provide any meaningful dynamics for the later reaction stages. The Parton Cascade Model (PCM) [5] and related pQCD approaches [6] treat the initial state as incoherent parton configuration, but are very well suited for the pre-equilibrium phase and subsequent thermalization, leading to a QGP and hydrodynamic expansion. Microscopic degrees of freedom in this ansatz are quarks and gluons which are propagated according to a Boltzmann Equation with a collision term using leading order pQCD cross sections. Augmented with a cluster hadronization ansatz the PCM is applicable up to hadronization. The range of this model can be even further extended if it is combined with a hadronic cascade which treats the hadronic phase and freeze-out correctly [7]. Nuclear Fluid Dynamics (NFD, see e.g. [8,9]) is ideally suited for the QGP and hy-drodynamic expansion reaction phase, but breaks down in the later, dilute, stages of the reaction when the mean free paths of the hadrons become large and flavor degrees of freedom are important. The reach of NFD can also be extended by combining it with a microscopic hadronic cascade model – this kind of hybrid approach (dubbed hydro plus micro) was pioneered in [10] and has been now also taken up by other groups [11]. It is to date the most successful approach for describing the soft …