Hadronization Revisited: the Dynamics behind Hadro-chemical Equilibrium

The multiplicities of hadronic species, from pions to omega hyperons created both in elementary, and in highly relativistic nucleus-nucleus collisions, are known to exhibit a pattern that is very well reproduced within the framework of the statistical model as applied, in its canonical form, to the former class of reactions, and in the grand canonical version to the latter. To understand the origin(s) of this apparent equilibrium we revisit, first, the hadronization models developed for e+e− annihilation to hadrons via partonic jet fragmentation that lead to a color pre-confinement state, occuring at the end of the perturbative QCD DGLAP evolution, which lends itself to color singlet formation that is then recast in terms of massive singlet clusters, in a non perturbative model. These clusters decay into the known hadron/resonance spectrum, the eventual hadronic species yield distribution thus being seen as the result of a perturbative evolution toward potential color singlet ”mass” distributions that are subsequently acquiring excited non perturbative hadronic condensate structures, that finally decay statistically (under phase space governance) to on-shell hadrons/resonances. Thus the success of a classical canonical ensemble description is understood as the consequence of various stochastic elements occuring in the dynamical evolution that ends with hadronic freeze-out. Turning to hadronization in relativistic nucleus-nucleus collisions we take advantage of the above singlet cluster pre-hadronization picture. These clusters are spatially isolated objects in the jet hadronization evolution, but with given extended spatial size, of the order of 1 fm. Lacking a detailed model of partonic transport evolution in highly relativistic A+A collisions we assume that hadronization occurs from similar color singlet clusters that will, however, overlap spatially owing to the extreme overall energy density. An ensuing cluster overlap coherence may be symbolically understood by means of a percolation model. Cluster overlap increases with √ s, A and collision centrality. In the limit of an extended coherent volume, hadronization is free of local quantum number constraints. After super-cluster decay the yield distribution is captured by the grand canonical version of the statistical equilibrium model which features strangeness saturation (large volume limit).