7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
315
between the two arising parton fields (opposing each other in longitudinal phase
space) should lead to a pile-up of partonic energy density centered at mid-rapidity
(the longitudinal coordinate of the overall center of mass). Due to this mutual
stopping down of the initial target and projectile parton fragmentation showers, and
from the concurrent decrease of parton virtuality (with decreasing average square
momentum transfer Q 2 ) there results a slowdown of the time scales governing the
dynamical evolution. Equilibrium could be approached here, the system “lands”
on the T , μ plane of Fig. 7.1, at temperatures of about 300 and 200 MeV at
top RHIC and top SPS energy, respectively. The third step, system expansion and
decay, thus occurs from well above the QCD parton-hadron boundary line. Hadrons
and hadronic resonances then form, which decouple swiftly from further inelastic
transmutation so that their yield ratios become stationary (“frozen-out”). A final
expansion period dilutes the system to a degree such that strong interaction ceases
all together.
In order to verify in detail this qualitative overall model, and to ascertain the
existence (and to study the properties) of the different states of QCD that are populated in sequence, one seeks observable physics quantities that convey information
imprinted during distinct stages of the dynamical evolution, and “freezing-out”
without significant obliteration by subsequent stages. Ordered in sequence of their
formation in the course of the dynamics, the most relevant such observables are
briefly characterized below:
1. Suppression of J// and Y production by Debye-screening in the QGP. These
vector mesons result from primordial, pQCD production of cc and bb pairs
that would hadronize unimpeded in elementary collisions but are broken up if
immersed into a npQCD deconfined QGP, at certain characteristic temperature
thresholds.
2. Suppression of dijets which arise from primordial qq pair production fragmenting into partonic showers (jets) in vacuum but being attenuated by QGP-medium
induced gluonic bremsstrahlung: Jet quenching in A+A collisions.
a. A variant of this: any primordial hard parton suffers a high, specific loss of
energy when traversing a deconfined medium: High p T suppression in A+A
collisions.
3. Hydrodynamic collective motion develops with the onset of (local) thermal
equilibrium. It is created by partonic pressure gradients that reflect the initial collisional impact geometry via non-isotropies in particle emission called “directed”
and “elliptic” flow. The latter reveals properties of the QGP, seen here as an ideal
partonic fluid.
a. Radial hydrodynamical expansion flow (“Hubble expansion”) is a variant of
the above that occurs in central, head on collisions with cylinder symmetry, as
a consequence of an isentropic expansion. It should be sensitive to the mixed
phase conditions characteristic of a first order parton-hadron phase transition.
315
between the two arising parton fields (opposing each other in longitudinal phase
space) should lead to a pile-up of partonic energy density centered at mid-rapidity
(the longitudinal coordinate of the overall center of mass). Due to this mutual
stopping down of the initial target and projectile parton fragmentation showers, and
from the concurrent decrease of parton virtuality (with decreasing average square
momentum transfer Q 2 ) there results a slowdown of the time scales governing the
dynamical evolution. Equilibrium could be approached here, the system “lands”
on the T , μ plane of Fig. 7.1, at temperatures of about 300 and 200 MeV at
top RHIC and top SPS energy, respectively. The third step, system expansion and
decay, thus occurs from well above the QCD parton-hadron boundary line. Hadrons
and hadronic resonances then form, which decouple swiftly from further inelastic
transmutation so that their yield ratios become stationary (“frozen-out”). A final
expansion period dilutes the system to a degree such that strong interaction ceases
all together.
In order to verify in detail this qualitative overall model, and to ascertain the
existence (and to study the properties) of the different states of QCD that are populated in sequence, one seeks observable physics quantities that convey information
imprinted during distinct stages of the dynamical evolution, and “freezing-out”
without significant obliteration by subsequent stages. Ordered in sequence of their
formation in the course of the dynamics, the most relevant such observables are
briefly characterized below:
1. Suppression of J// and Y production by Debye-screening in the QGP. These
vector mesons result from primordial, pQCD production of cc and bb pairs
that would hadronize unimpeded in elementary collisions but are broken up if
immersed into a npQCD deconfined QGP, at certain characteristic temperature
thresholds.
2. Suppression of dijets which arise from primordial qq pair production fragmenting into partonic showers (jets) in vacuum but being attenuated by QGP-medium
induced gluonic bremsstrahlung: Jet quenching in A+A collisions.
a. A variant of this: any primordial hard parton suffers a high, specific loss of
energy when traversing a deconfined medium: High p T suppression in A+A
collisions.
3. Hydrodynamic collective motion develops with the onset of (local) thermal
equilibrium. It is created by partonic pressure gradients that reflect the initial collisional impact geometry via non-isotropies in particle emission called “directed”
and “elliptic” flow. The latter reveals properties of the QGP, seen here as an ideal
partonic fluid.
a. Radial hydrodynamical expansion flow (“Hubble expansion”) is a variant of
the above that occurs in central, head on collisions with cylinder symmetry, as
a consequence of an isentropic expansion. It should be sensitive to the mixed
phase conditions characteristic of a first order parton-hadron phase transition.