430
R. Stock
initialization, hydrodynamic expansion, hadronization and final hadron/resonance
“gas” expansion. We shall focus on the two characteristic parameters of the QCD
plasma medium, the specific viscosity η/s and the transport coefficient ˆ
q, as
revealed by anisotropic flow and hadron/jet suppression, respectively. Then we turn
to charmonium and bottonium suppression in the QCD medium, paying special
attention to charmed meson statistical equilibrium hadronization. This leads us to
hadronization and the information we have on the QCD phase diagram. These topics
are all in full development since RHIC’s first decade of operation, and have been
introduced in the preceding review article. Then we turn to a couple of more recent
topics, e.g. small systems analysis, formation of light nuclei and antinuclei, critical
point searches and the possible role of (now observable) neutron star mergers to
unravel the EOS of cold hadronic matter.
7.9.2 Reaction Dynamics
Whereas, at the SPS energies, a central P b + P b collision featured some fraction of
the collisional system already expanding toward hadronization at the time where
the last participant nucleons just experienced their first collisions, the various
evolutional stages are clearly separated from top RHIC energy onward. With
Lorentz contraction factors in the hundreds to thousands domain, the primordial
collisional system longitudinal size is less than a nucleon radius such that all
primordial interactions occur “at once” so that after about 0.5 fm/c the initialization
processes can be expected to settle down toward local equilibrium. This creates a
primordial fireball that still features a highly clumpy energy density distribution.
This distribution fluctuates from event to event due to a number of influences
stemming from remaining impact parameter fluctuation, instantaneous density fluctuations within the average projectile-target Woods Saxon nucleon position/density
profile (which are thus getting “photographed”), and by the presence or absence
of colour saturation processes [303]. The latter can go along with the formation
of a Colour Glass Condensate State [304] which decays to a parton system at the
end of the initialization period, thus creating a particular topology of the primordial
energy density distribution [305]. A model calculation of a zero impact parameter
Au + Au collision transverse energy density distribution was already shown in
Fig. 7.60, exhibiting pronounced clumpiness. This distribution is then, approximately, translated into the energy-momentum tensor that starts the next stage:
hydrodynamic expansion. The most remarkable feature of this evolution (see below)
is the occurrence of a nearly ideal hydro-flow dynamics which approximatively
preserves information, the better the lower dissipative processes like shear viscosity
turn out to be. Thus, most remarkably, the eventwise fluctuating primordial fireball
profile becomes measurable after flow decoupling, modulo the strength of shear
viscosity. Two of the objects of desire in this physics [306]!
The hydrodynamic evolution stage is thus, theoretically, well separable from the
initialization period. The transition is, in itself, of course a matter of theoretical
R. Stock
initialization, hydrodynamic expansion, hadronization and final hadron/resonance
“gas” expansion. We shall focus on the two characteristic parameters of the QCD
plasma medium, the specific viscosity η/s and the transport coefficient ˆ
q, as
revealed by anisotropic flow and hadron/jet suppression, respectively. Then we turn
to charmonium and bottonium suppression in the QCD medium, paying special
attention to charmed meson statistical equilibrium hadronization. This leads us to
hadronization and the information we have on the QCD phase diagram. These topics
are all in full development since RHIC’s first decade of operation, and have been
introduced in the preceding review article. Then we turn to a couple of more recent
topics, e.g. small systems analysis, formation of light nuclei and antinuclei, critical
point searches and the possible role of (now observable) neutron star mergers to
unravel the EOS of cold hadronic matter.
7.9.2 Reaction Dynamics
Whereas, at the SPS energies, a central P b + P b collision featured some fraction of
the collisional system already expanding toward hadronization at the time where
the last participant nucleons just experienced their first collisions, the various
evolutional stages are clearly separated from top RHIC energy onward. With
Lorentz contraction factors in the hundreds to thousands domain, the primordial
collisional system longitudinal size is less than a nucleon radius such that all
primordial interactions occur “at once” so that after about 0.5 fm/c the initialization
processes can be expected to settle down toward local equilibrium. This creates a
primordial fireball that still features a highly clumpy energy density distribution.
This distribution fluctuates from event to event due to a number of influences
stemming from remaining impact parameter fluctuation, instantaneous density fluctuations within the average projectile-target Woods Saxon nucleon position/density
profile (which are thus getting “photographed”), and by the presence or absence
of colour saturation processes [303]. The latter can go along with the formation
of a Colour Glass Condensate State [304] which decays to a parton system at the
end of the initialization period, thus creating a particular topology of the primordial
energy density distribution [305]. A model calculation of a zero impact parameter
Au + Au collision transverse energy density distribution was already shown in
Fig. 7.60, exhibiting pronounced clumpiness. This distribution is then, approximately, translated into the energy-momentum tensor that starts the next stage:
hydrodynamic expansion. The most remarkable feature of this evolution (see below)
is the occurrence of a nearly ideal hydro-flow dynamics which approximatively
preserves information, the better the lower dissipative processes like shear viscosity
turn out to be. Thus, most remarkably, the eventwise fluctuating primordial fireball
profile becomes measurable after flow decoupling, modulo the strength of shear
viscosity. Two of the objects of desire in this physics [306]!
The hydrodynamic evolution stage is thus, theoretically, well separable from the
initialization period. The transition is, in itself, of course a matter of theoretical