7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
367
fit of Fig. 7.17 requires γ s = 0.66, a value typical of canonical multiplicity analysis
in p+p, p+p and e + e − annihilation collisions [109] at
√
s ≥ 30 GeV.
The above picture, of hadrochemical equilibrium resulting from the combined
stochastic features of QCD color neutralization by cluster formation, and subsequent
quantum mechanical decay to the on-shell hadron and resonance spectrum (under
phase space governance) lends itself to a straight forward extension to A+A
collisions. The essential new features, of grand canonical hadronization including
strangeness enhancement, should result from the fact that extended space-time
volumes of > > crit are formed in the course of primordial partonic shower
evolution, an overlap effect increasing both with
√
s and with the size of the
primordial interaction volume. As the volume of the elementary hadronization
clusters amounts to several fm 3 it is inevitable that the clusters coalesce, to form
extended “super-cluster” volumes prior to hadronization [120]. As these superclusters develop toward hadronization via non perturbative QCD dynamics, it is
plausible to assume an overall quantum mechanical coherence to arise over the
entire extended volume, which will thus decay to hadrons under global quantum
number conservation, the decay products thus modeled by the GC ensemble.
Our expectation that space-time coalescence of individual hadronization clusters
will lead to a global, quantum mechanically coherent extended super-cluster
volume, that decays under phase space dominance, appears as an analogy to the
dynamics and quantum mechanics governing low energy nuclear fission from
a preceding “compound nucleus” [126]. Note that the observation of a smooth
transition from canonical strangeness suppression to grand canonical saturation
(Figs. 7.29, 7.30) lends further support to the above picture of a percolative growth
[120] of the volume that is about to undergo hadronization.
An extended, coherent quark gluon plasma state would, of course, represent
an ideal example of such a volume [127] and, in fact, we could imagine that the
spatial extension of the plasma state results from a percolative overlap of primordial
zones of high energy density, which becomes more prominent with increasing
√
s
and N part . A QGP state preceding hadronization will thus lead to all the observed
features. However, to be precise: the hadronizing QCD system of extended matter
decaying quantum coherently, could still be a non-equilibrium precursor of the ideal
equilibrium QGP, because we have seen above that hadrochemical equilibrium also
occurs in e + e − annihilation, where no partonic equilibrium exists. It gets established
in the course of hadronization, irrespective of the degree of equilibrium prevailing
in the preceding partonic phase.
7.3.4 Hadronization vs. Rapidity and
√
s
We have argued in Sect. 7.3.1 that, at relatively low
√
s, the total rapidity gap
y does not significantly exceed the natural thermal rapidity spreading width
i ≈ 2.35 (T /m i ) 1/2 of a single, isotropically decaying fireball, centered at
mid-rapidity and emitting hadrons of mass m i [110]. However, this procedure
367
fit of Fig. 7.17 requires γ s = 0.66, a value typical of canonical multiplicity analysis
in p+p, p+p and e + e − annihilation collisions [109] at
√
s ≥ 30 GeV.
The above picture, of hadrochemical equilibrium resulting from the combined
stochastic features of QCD color neutralization by cluster formation, and subsequent
quantum mechanical decay to the on-shell hadron and resonance spectrum (under
phase space governance) lends itself to a straight forward extension to A+A
collisions. The essential new features, of grand canonical hadronization including
strangeness enhancement, should result from the fact that extended space-time
volumes of > > crit are formed in the course of primordial partonic shower
evolution, an overlap effect increasing both with
√
s and with the size of the
primordial interaction volume. As the volume of the elementary hadronization
clusters amounts to several fm 3 it is inevitable that the clusters coalesce, to form
extended “super-cluster” volumes prior to hadronization [120]. As these superclusters develop toward hadronization via non perturbative QCD dynamics, it is
plausible to assume an overall quantum mechanical coherence to arise over the
entire extended volume, which will thus decay to hadrons under global quantum
number conservation, the decay products thus modeled by the GC ensemble.
Our expectation that space-time coalescence of individual hadronization clusters
will lead to a global, quantum mechanically coherent extended super-cluster
volume, that decays under phase space dominance, appears as an analogy to the
dynamics and quantum mechanics governing low energy nuclear fission from
a preceding “compound nucleus” [126]. Note that the observation of a smooth
transition from canonical strangeness suppression to grand canonical saturation
(Figs. 7.29, 7.30) lends further support to the above picture of a percolative growth
[120] of the volume that is about to undergo hadronization.
An extended, coherent quark gluon plasma state would, of course, represent
an ideal example of such a volume [127] and, in fact, we could imagine that the
spatial extension of the plasma state results from a percolative overlap of primordial
zones of high energy density, which becomes more prominent with increasing
√
s
and N part . A QGP state preceding hadronization will thus lead to all the observed
features. However, to be precise: the hadronizing QCD system of extended matter
decaying quantum coherently, could still be a non-equilibrium precursor of the ideal
equilibrium QGP, because we have seen above that hadrochemical equilibrium also
occurs in e + e − annihilation, where no partonic equilibrium exists. It gets established
in the course of hadronization, irrespective of the degree of equilibrium prevailing
in the preceding partonic phase.
7.3.4 Hadronization vs. Rapidity and
√
s
We have argued in Sect. 7.3.1 that, at relatively low
√
s, the total rapidity gap
y does not significantly exceed the natural thermal rapidity spreading width
i ≈ 2.35 (T /m i ) 1/2 of a single, isotropically decaying fireball, centered at
mid-rapidity and emitting hadrons of mass m i [110]. However, this procedure