Hadron Formation in High Energy Elementary and Nuclear Collisions

We consider the dynamical origin of the apparent statistical equilibrium that governs the yields, and yield ratios, of all hadron and resonance species (consisting of the three light quark flavours) produced in nucleus-nucleus collisions from AGS via SPS to RHIC energies 5<= √ {s} <= 200 GeV. This hadro-chemical equilibrium state is well described, overall, by the grand canonical, quasi-classical Gibbs ensemble of all corresponding hadrons and resonances. In order to pin down the stochastic elements, featured by the dynamical evolution prior to hadron formation and hadronic "chemical" (i.e. species) freeze-out, and determining the eventual equilibrium state, we concentrate on the high energy domain, √ {s} >= 20 GeV, where a model of primordial perturbative QCD partonic shower evolution appears plausible. For guidance concerning a hadronization model we revisit the QCD description of jet-induced hadron formation in e⁺e^- annihilation at LEP energy. At the end of the pQCD partonic shower evolution a stage of color neutralization and flavour recombination leads to transition into non perturbative QCD clusters or strings, that decay to hadrons/resonances under phase space dominance. The combination of stochastic shower multiplication and cluster decay to the phase space defined by the hadron/resonancemass and spin spectrum results in a hadronization output featuring statistical equilibrium of the species, which is well described by the canonical Gibbs ensemble. We then assume that hadronization in A+A collisions occurs from a similar stage of singlet cluster formation. However, owing to the extreme overall energy density these clusters should overlap spatially, giving rise to extended super-cluster formation, increasing with √ {s}, A and collision centrality. In the limit of an extended volume decaying coherently, hadronization is free of local quantum number conservation constraints. This leads to strangeness enhancement and explains the success of a grand canonical description.