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R. Stock
4. Hadronic “chemical” freeze-out fixes the abundance ratios of the hadronic
species into an equilibrium distribution. Occurring very close to, or at hadronization, it reveals the dynamical evolution path in the [T , μ B ] plane and determines
the critical temperature and density of QCD. The yield distributions in A+A
collisions show a dramatic strangeness enhancement effect, characteristic of an
extended QCD medium.
5. Fluctuations, from one collision event to another (and even within a single given
event) can be quantified in A+A collisions due to the high charged hadron
multiplicity density (of up to 600 per rapidity unit at top RHIC energy). Such
event-by-event (Debye) fluctuations of pion rapidity density and mean transverse
momentum (event “temperature”), as well as event-wise fluctuations of the
strange to non-strange hadron abundance ratio (may) reflect the existence and
position of the conjectured critical point of QCD (Fig. 7.1).
6. Two particle Bose-Einstein-Correlations are the analog of the Hanbury-Brown,
Twiss (HBT) effect of quantum optics. They result from the last interaction
experienced by hadrons, i.e. from the global decoupling stage. Owing to a near
isentropic hadronic expansion they reveal information on the overall space-timedevelopment of the “fireball” evolution.
In an overall view the first group of observables (1 to 2a) is anchored in
established pQCD physics that is well known from theoretical and experimental
analysis of elementary collisions (e + e − annihilation, pp and pp data). In fact, the
first generation of high Q 2 baryon collisions, occurring at the microscopic level
in A+A collisions, should closely resemble such processes. However, their primary
partonic products do not escape into pQCD vacuum but get attenuated by interaction
with the concurrently developing extended high density medium, thus serving as
diagnostic tracer probes of that state. The remaining observables capture snapshots
of the bulk matter medium itself. After initial equilibration we may confront elliptic
flow data with QCD during the corresponding partonic phase of the dynamical
evolution employing thermodynamic [21] and hydrodynamic [22] models of a high
temperature parton plasma. The hydro-model stays applicable well into the hadronic
phase. Hadron formation (confinement) occurs in between these phases (at about
5 μs time in the cosmological evolution). In fact relativistic nuclear collision data
may help to finally pin down the mechanism(s) of this fascinating QCD process
[23–25] as we can vary the conditions of its occurrence, along the parton-hadron
phase separation line of Fig. 7.1, by proper choice of collisional energy
√
s, and
system size A, while maintaining the overall conditions of an extended imbedding
medium of high energy density within which various patterns [9–11, 15, 16] of the
hadronization phase transition may establish. The remaining physics observables
(3a, 5 and 6 above) essentially provide for auxiliary information about the bulk
matter system as it traverses (and emerges from) the hadronization stage, with
special emphasis placed on manifestations of the conjectured critical point.
The present review will briefly cover each of the above physics observables in a
separate chapter, beginning with the phenomena of confinement and hadronization
(Sect. 7.3), then to turn to the preceding primordial dynamics, e.g. to elliptical
R. Stock
4. Hadronic “chemical” freeze-out fixes the abundance ratios of the hadronic
species into an equilibrium distribution. Occurring very close to, or at hadronization, it reveals the dynamical evolution path in the [T , μ B ] plane and determines
the critical temperature and density of QCD. The yield distributions in A+A
collisions show a dramatic strangeness enhancement effect, characteristic of an
extended QCD medium.
5. Fluctuations, from one collision event to another (and even within a single given
event) can be quantified in A+A collisions due to the high charged hadron
multiplicity density (of up to 600 per rapidity unit at top RHIC energy). Such
event-by-event (Debye) fluctuations of pion rapidity density and mean transverse
momentum (event “temperature”), as well as event-wise fluctuations of the
strange to non-strange hadron abundance ratio (may) reflect the existence and
position of the conjectured critical point of QCD (Fig. 7.1).
6. Two particle Bose-Einstein-Correlations are the analog of the Hanbury-Brown,
Twiss (HBT) effect of quantum optics. They result from the last interaction
experienced by hadrons, i.e. from the global decoupling stage. Owing to a near
isentropic hadronic expansion they reveal information on the overall space-timedevelopment of the “fireball” evolution.
In an overall view the first group of observables (1 to 2a) is anchored in
established pQCD physics that is well known from theoretical and experimental
analysis of elementary collisions (e + e − annihilation, pp and pp data). In fact, the
first generation of high Q 2 baryon collisions, occurring at the microscopic level
in A+A collisions, should closely resemble such processes. However, their primary
partonic products do not escape into pQCD vacuum but get attenuated by interaction
with the concurrently developing extended high density medium, thus serving as
diagnostic tracer probes of that state. The remaining observables capture snapshots
of the bulk matter medium itself. After initial equilibration we may confront elliptic
flow data with QCD during the corresponding partonic phase of the dynamical
evolution employing thermodynamic [21] and hydrodynamic [22] models of a high
temperature parton plasma. The hydro-model stays applicable well into the hadronic
phase. Hadron formation (confinement) occurs in between these phases (at about
5 μs time in the cosmological evolution). In fact relativistic nuclear collision data
may help to finally pin down the mechanism(s) of this fascinating QCD process
[23–25] as we can vary the conditions of its occurrence, along the parton-hadron
phase separation line of Fig. 7.1, by proper choice of collisional energy
√
s, and
system size A, while maintaining the overall conditions of an extended imbedding
medium of high energy density within which various patterns [9–11, 15, 16] of the
hadronization phase transition may establish. The remaining physics observables
(3a, 5 and 6 above) essentially provide for auxiliary information about the bulk
matter system as it traverses (and emerges from) the hadronization stage, with
special emphasis placed on manifestations of the conjectured critical point.
The present review will briefly cover each of the above physics observables in a
separate chapter, beginning with the phenomena of confinement and hadronization
(Sect. 7.3), then to turn to the preceding primordial dynamics, e.g. to elliptical