7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
319
obvious by Lattice QCD calculations for extended matter [40]. Seen in retrospect
one obviously cannot defend a picture of point like quarks (with “current” masses)
at Q 2 ≤ 0.2 GeV 2 where size scales of 0.5 to 1 fm must play a dominating role.
An analytic QCD description of deconfinement does not exist. For heavy
quarkonia, cc (J//) and bb (Y ) deconfinement in partonic matter, Matsui and Satz
proposed [41] a Debye screening mechanism, caused by the high spatial density of
free color carriers, that removes the confining long range potential as T increases
toward about 2 T c , an effect reproduced by modern lattice QCD [42]. However, light
hadron deconfinement cannot be understood with a non-relativistic potential model.
Such critical remarks not withstanding, we shall demonstrate in Sects. 7.3–7.6 that
the very existence, and also crucial properties of the QGP can in fact be inferred
from experiment, and be confronted with corresponding predictions of recent lattice
QCD theory.
Our present level of an initial understanding of the phase diagram of QCD matter
(Fig. 7.1), is the result of a steady development of both experiment and theory, that
began about three decades ago, deriving initial momentum from the Bevalac physics
at LBL which motivated—along with the developing formulation of the quark gluon
plasma research goals—a succession of experimental facilities progressing toward
higher
√
s. Beginning with the AGS at BNL ( 28 Si and 197 Au beams with
√
s ≤
5 GeV), the next steps were taken at the CERN SPS (
√
s from 6 to 20 GeV; 16 O,
32 S, 208 Pb beams), and at the Relativistic Heavy Ion Collider RHIC (the first facility
constructed explicitly for nuclear collisions) which offers beams of 64 Cu and 197 Au
at 20 ≤
√
s ≤ 200 GeV. A final, gigantic step in energy will be taken 2008 with the
CERN Large Hadron Collider: 208 Pb beams at
√
s = 5.5 TeV.
7.2 Bulk Hadron Production in A+A Collisions
In this section we take an overall look at bulk hadron production in nucleusnucleus collisions. In view of the high total c.m. energies involved at e.g. top SPS
(E tot
cm ≈ 3.3 TeV) and top RHIC (38 TeV) energies, in central Pb+Pb (SPS) and
Au+Au (RHIC) collisions, one can expect an extraordinarily high spatial density of
produced particles. Thus, as an overall idea of analysis, one will try to relate the
observed flow of energy into transverse and longitudinal phase space and particle
species to the high energy density contained in the primordial interaction volume,
thus to infer about its contained matter. The typical experimental patterns of such
collisions, both in collider mode at RHIC and in a fixed target configuration at the
SPS, are illustrated in Fig. 7.2 which shows a fractional view of the total distribution
of charged particles (about 4000 and 1600, respectively) within the tracking volume
of the STAR and NA49 experiments.
Most of these tracks correspond to “thermal” pions (p T up to 2 GeV) and, in
general, such thermal hadrons make up for about 95% of the observed multiplicity:
the bulk of hadron production. Their distributions in phase space will be illustrated
in the subsections below. This will lead to a first insight into the overall reaction
319
obvious by Lattice QCD calculations for extended matter [40]. Seen in retrospect
one obviously cannot defend a picture of point like quarks (with “current” masses)
at Q 2 ≤ 0.2 GeV 2 where size scales of 0.5 to 1 fm must play a dominating role.
An analytic QCD description of deconfinement does not exist. For heavy
quarkonia, cc (J//) and bb (Y ) deconfinement in partonic matter, Matsui and Satz
proposed [41] a Debye screening mechanism, caused by the high spatial density of
free color carriers, that removes the confining long range potential as T increases
toward about 2 T c , an effect reproduced by modern lattice QCD [42]. However, light
hadron deconfinement cannot be understood with a non-relativistic potential model.
Such critical remarks not withstanding, we shall demonstrate in Sects. 7.3–7.6 that
the very existence, and also crucial properties of the QGP can in fact be inferred
from experiment, and be confronted with corresponding predictions of recent lattice
QCD theory.
Our present level of an initial understanding of the phase diagram of QCD matter
(Fig. 7.1), is the result of a steady development of both experiment and theory, that
began about three decades ago, deriving initial momentum from the Bevalac physics
at LBL which motivated—along with the developing formulation of the quark gluon
plasma research goals—a succession of experimental facilities progressing toward
higher
√
s. Beginning with the AGS at BNL ( 28 Si and 197 Au beams with
√
s ≤
5 GeV), the next steps were taken at the CERN SPS (
√
s from 6 to 20 GeV; 16 O,
32 S, 208 Pb beams), and at the Relativistic Heavy Ion Collider RHIC (the first facility
constructed explicitly for nuclear collisions) which offers beams of 64 Cu and 197 Au
at 20 ≤
√
s ≤ 200 GeV. A final, gigantic step in energy will be taken 2008 with the
CERN Large Hadron Collider: 208 Pb beams at
√
s = 5.5 TeV.
7.2 Bulk Hadron Production in A+A Collisions
In this section we take an overall look at bulk hadron production in nucleusnucleus collisions. In view of the high total c.m. energies involved at e.g. top SPS
(E tot
cm ≈ 3.3 TeV) and top RHIC (38 TeV) energies, in central Pb+Pb (SPS) and
Au+Au (RHIC) collisions, one can expect an extraordinarily high spatial density of
produced particles. Thus, as an overall idea of analysis, one will try to relate the
observed flow of energy into transverse and longitudinal phase space and particle
species to the high energy density contained in the primordial interaction volume,
thus to infer about its contained matter. The typical experimental patterns of such
collisions, both in collider mode at RHIC and in a fixed target configuration at the
SPS, are illustrated in Fig. 7.2 which shows a fractional view of the total distribution
of charged particles (about 4000 and 1600, respectively) within the tracking volume
of the STAR and NA49 experiments.
Most of these tracks correspond to “thermal” pions (p T up to 2 GeV) and, in
general, such thermal hadrons make up for about 95% of the observed multiplicity:
the bulk of hadron production. Their distributions in phase space will be illustrated
in the subsections below. This will lead to a first insight into the overall reaction