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R. Stock
that could have served as seedlings of galactic cluster formation [18]. However, it
needs to be stressed that the conjectured order of phase transformation, occurring
along the parton-hadron phase boundary line, has not been unambiguously confirmed by experiment, as of now.
On the other hand, the position of the QCD phase boundary at low μ B has,
in fact, been located by the hadronization points in the T , μ B plane that are also
illustrated in Fig. 7.1. They are obtained from statistical model analysis [19] of the
various hadron multiplicities created in nucleus-nucleus collisions, which results
in a [T , μ B ] determination at each incident energy, which ranges from SIS via
AGS and SPS to RHIC energies, i.e. 3 ≤
√
s ≤ 200 GeV. Toward low μ B these
hadronic freeze-out points merge with the lattice QCD parton-hadron coexistence
line: hadron formation coincides with hadronic species freeze-out. These points
also indicate the μ B domain of the phase diagram which is accessible to relativistic
nuclear collisions. The domain at μ B ≥ 1.5 GeV which is predicted to be in a further
new phase of QCD featuring color-flavor locking and color superconductivity [20]
will probably be accessible only to astrophysical observation.
One may wonder how states and phases of matter in thermodynamical
equilibrium—as implied by a description in grand canonical variables—can be
sampled via the dynamical evolution of relativistic nuclear collisions. Employing
heavy nuclei, A ≈ 200, as projectiles/targets or in colliding beams (RHIC, LHC),
transverse dimensions of the primordial interaction volume do not exceed about
8 fm, and strong interaction ceases after about 20 fm/c. We shall devote an entire
later section to the aspects of equilibrium (Sect. 7.2.5) but note, for now, that
the time and dimension scale of primordial perturbative QCD interaction at the
microscopic partonic level amounts to subfractions of 1 fm/c, the latter scale,
however, being representative of non perturbative processes (confinement, “string”
formation etc.). The A+A fireball size thus exceeds, by far, the elementary non
perturbative scale. An equilibrium quark gluon plasma represents an extended
non-perturbative QCD object, and the question whether its relaxation time scale
can be provided by the expansion time scale of an A+A collision, needs careful
examination. Reassuringly, however, the hadrons that are supposedly created from
such a preceding non-perturbative QGP phase at top SPS and RHIC energy, do in
fact exhibit perfect hadrochemical equilibrium, the derived [T , μ B ] values [19] thus
legitimately appearing in the phase diagram, Fig. 7.1.
In the present review we will order the physics observables to be treated,
in sequence of their origin from successive stages that characterize the overall
dynamical evolution of a relativistic nucleus-nucleus collision. In rough outline
this evolution can be seen to proceed in three major steps. An initial period of
matter compression and heating occurs in the course of interpenetration of the
projectile and target baryon density distributions. Inelastic processes occurring at
the microscopic level convert initial beam longitudinal energy to new internal and
transverse degrees of freedom, by breaking up the initial baryon structure functions.
Their partons thus acquire virtual mass, populating transverse phase space in the
course of inelastic perturbative QCD shower multiplication. This stage should be
far from thermal equilibrium, initially. However, in step two, inelastic interaction
R. Stock
that could have served as seedlings of galactic cluster formation [18]. However, it
needs to be stressed that the conjectured order of phase transformation, occurring
along the parton-hadron phase boundary line, has not been unambiguously confirmed by experiment, as of now.
On the other hand, the position of the QCD phase boundary at low μ B has,
in fact, been located by the hadronization points in the T , μ B plane that are also
illustrated in Fig. 7.1. They are obtained from statistical model analysis [19] of the
various hadron multiplicities created in nucleus-nucleus collisions, which results
in a [T , μ B ] determination at each incident energy, which ranges from SIS via
AGS and SPS to RHIC energies, i.e. 3 ≤
√
s ≤ 200 GeV. Toward low μ B these
hadronic freeze-out points merge with the lattice QCD parton-hadron coexistence
line: hadron formation coincides with hadronic species freeze-out. These points
also indicate the μ B domain of the phase diagram which is accessible to relativistic
nuclear collisions. The domain at μ B ≥ 1.5 GeV which is predicted to be in a further
new phase of QCD featuring color-flavor locking and color superconductivity [20]
will probably be accessible only to astrophysical observation.
One may wonder how states and phases of matter in thermodynamical
equilibrium—as implied by a description in grand canonical variables—can be
sampled via the dynamical evolution of relativistic nuclear collisions. Employing
heavy nuclei, A ≈ 200, as projectiles/targets or in colliding beams (RHIC, LHC),
transverse dimensions of the primordial interaction volume do not exceed about
8 fm, and strong interaction ceases after about 20 fm/c. We shall devote an entire
later section to the aspects of equilibrium (Sect. 7.2.5) but note, for now, that
the time and dimension scale of primordial perturbative QCD interaction at the
microscopic partonic level amounts to subfractions of 1 fm/c, the latter scale,
however, being representative of non perturbative processes (confinement, “string”
formation etc.). The A+A fireball size thus exceeds, by far, the elementary non
perturbative scale. An equilibrium quark gluon plasma represents an extended
non-perturbative QCD object, and the question whether its relaxation time scale
can be provided by the expansion time scale of an A+A collision, needs careful
examination. Reassuringly, however, the hadrons that are supposedly created from
such a preceding non-perturbative QGP phase at top SPS and RHIC energy, do in
fact exhibit perfect hadrochemical equilibrium, the derived [T , μ B ] values [19] thus
legitimately appearing in the phase diagram, Fig. 7.1.
In the present review we will order the physics observables to be treated,
in sequence of their origin from successive stages that characterize the overall
dynamical evolution of a relativistic nucleus-nucleus collision. In rough outline
this evolution can be seen to proceed in three major steps. An initial period of
matter compression and heating occurs in the course of interpenetration of the
projectile and target baryon density distributions. Inelastic processes occurring at
the microscopic level convert initial beam longitudinal energy to new internal and
transverse degrees of freedom, by breaking up the initial baryon structure functions.
Their partons thus acquire virtual mass, populating transverse phase space in the
course of inelastic perturbative QCD shower multiplication. This stage should be
far from thermal equilibrium, initially. However, in step two, inelastic interaction