The hadronization of an expanding partonic fireball is studied within the parton-hadron-string-dynamics (PHSD) approach, which is based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. Apart from strong parton interactions, the expansion and development of collective flow is driven by strong gradients in the parton mean fields. An analysis of the elliptic flow $v_2$ demonstrates a linear correlation with the spatial eccentricity ε as in ideal hydrodynamics. The hadronization occurs by quark-antiquark fusion or three-quark/three-antiquark recombination, which is described by covariant transition rates. Since the dynamical quarks become very massive, the formed resonant “pre-hadronic” color-dipole states ($q\overline{q}$ or $\mathrm{qqq}$) are of high invariant mass, too, and sequentially decay to the ground-state meson and baryon octets increasing the total entropy. This solves the entropy problem in hadronization in a natural way. The resulting particle ratios turn out to be in line with those from a grand-canonical partition function at temperature $T\approx 170$ MeV rather independent from the initial temperature and indicate an approximate strangeness equilibration.

• Received 6 April 2008

DOI:https://doi.org/10.1103/PhysRevC.78.034919