7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
425
In the light of the considerations above, the plateau might turn out to reflect a critical
point focusing effect.
7.8 Summary
We have seen that many physics observables in relativistic nucleus-nucleus collisions can, at RHIC energy
√
s = 200 GeV, be related to the primordial dynamical
phase, from initial QCD parton shower formation to local momentum space
equilibration. The time interval is 0.35 to 0.65 fm/c. This domain can be investigated
at RHIC due to the short interpenetration time, of 0.15 fm/c. From among the bulk
hadron production signals, total and midrapidity charged particle rapidity densities,
d N ch /d y, reflect the primordial parton saturation phenomenon [72]. It leads to an
unexpectedly slow increase of multiplicity with fireball participant number N part ,
and with
√
s. This observation signals the onset of nonperturbative QCD, a coherent
shower multiplication by multigluon coherence [62, 65, 70, 71, 75]. It is expected
to be even more dominant in LHC Pb+Pb collisions at 5.5 TeV. Furthermore,
elliptic flow, a collective bulk hadron emission anisotropy, also originates from
the primordial, nonisotropic spatial density profile of shower-produced partons
[93, 96]. A hydrodynamic evolution sets in at t < 1 fm/c, implying the existence
of a primordial equation of state (EOS) for partonic matter in local equilibrium.
Moreover, the experimental elliptic flow data at RHIC are well described by “ideal
fluid” hydrodynamics, i.e. by a very small shear viscosity η and a small mean free
path λ [94, 95].
These observations indicate the existence of a (local) equilibrium state at very
early times in the bulk parton matter stage of the dynamical evolution. This quarkgluon plasma (QGP) state of nonperturbative QCD was predicted by QCD lattice
theory [11]. In Au+Au collisions at RHIC energy this state appears to be realized
under the following conditions [61].
The energy density amounts to 6–15 GeV/fm 3 , far above the parton-hadron
QCD confinement region at = 1 GeV/fm 3 , and at about 55 times the density
of nuclear ground state matter, ρ 0 = 0.14 GeV/fm 3 . Translating to macroscopic
units we are dealing with a matter density of about 1.3 · 10 19 kg/m 3 , the density
prevailing in the picosecond era of the cosmological evolution. The corresponding
temperature amounts to T = 300–330 MeV (about 3.6 · 10 12 K), far above the
“Hagedorn limit” [38] for the hadronic phase (T H = 165 MeV). From an analysis
of the production ratios of the various hadronic species (from pions to Omega
hyperons) arising from this primordial partonic state at hadronization, the statistical
hadronization model (SHM) determines its baryo-chemical potential [108], μ B =
20 MeV at RHIC energy. This value indicates that one is already close to the nearzero net baryon number conditions, μ B ≈ 0, prevailing in the corresponding big
bang evolution, where the density of particles exceeds that of antiparticles by a
fraction of about 10 −9 only.
425
In the light of the considerations above, the plateau might turn out to reflect a critical
point focusing effect.
7.8 Summary
We have seen that many physics observables in relativistic nucleus-nucleus collisions can, at RHIC energy
√
s = 200 GeV, be related to the primordial dynamical
phase, from initial QCD parton shower formation to local momentum space
equilibration. The time interval is 0.35 to 0.65 fm/c. This domain can be investigated
at RHIC due to the short interpenetration time, of 0.15 fm/c. From among the bulk
hadron production signals, total and midrapidity charged particle rapidity densities,
d N ch /d y, reflect the primordial parton saturation phenomenon [72]. It leads to an
unexpectedly slow increase of multiplicity with fireball participant number N part ,
and with
√
s. This observation signals the onset of nonperturbative QCD, a coherent
shower multiplication by multigluon coherence [62, 65, 70, 71, 75]. It is expected
to be even more dominant in LHC Pb+Pb collisions at 5.5 TeV. Furthermore,
elliptic flow, a collective bulk hadron emission anisotropy, also originates from
the primordial, nonisotropic spatial density profile of shower-produced partons
[93, 96]. A hydrodynamic evolution sets in at t < 1 fm/c, implying the existence
of a primordial equation of state (EOS) for partonic matter in local equilibrium.
Moreover, the experimental elliptic flow data at RHIC are well described by “ideal
fluid” hydrodynamics, i.e. by a very small shear viscosity η and a small mean free
path λ [94, 95].
These observations indicate the existence of a (local) equilibrium state at very
early times in the bulk parton matter stage of the dynamical evolution. This quarkgluon plasma (QGP) state of nonperturbative QCD was predicted by QCD lattice
theory [11]. In Au+Au collisions at RHIC energy this state appears to be realized
under the following conditions [61].
The energy density amounts to 6–15 GeV/fm 3 , far above the parton-hadron
QCD confinement region at = 1 GeV/fm 3 , and at about 55 times the density
of nuclear ground state matter, ρ 0 = 0.14 GeV/fm 3 . Translating to macroscopic
units we are dealing with a matter density of about 1.3 · 10 19 kg/m 3 , the density
prevailing in the picosecond era of the cosmological evolution. The corresponding
temperature amounts to T = 300–330 MeV (about 3.6 · 10 12 K), far above the
“Hagedorn limit” [38] for the hadronic phase (T H = 165 MeV). From an analysis
of the production ratios of the various hadronic species (from pions to Omega
hyperons) arising from this primordial partonic state at hadronization, the statistical
hadronization model (SHM) determines its baryo-chemical potential [108], μ B =
20 MeV at RHIC energy. This value indicates that one is already close to the nearzero net baryon number conditions, μ B ≈ 0, prevailing in the corresponding big
bang evolution, where the density of particles exceeds that of antiparticles by a
fraction of about 10 −9 only.