318
R. Stock
resonance—a collective density oscillation also called “breathing mode”—covered
only the parabolic minimum occurring in the related function of energy vs. density
at T = 0, = 0 .
The situation changed around 1970 due to the prediction made by W. Greiner et
al. [31] that nucleus-nucleus collisions, at relatively modest relativistic energies,
would result in shock compression. This mechanism promised to reach matter
densities far beyond those of a mere superposition (i.e. // 0 ≤ 2γ ) of initial
target and projectile densities. Coinciding in time, the newly developed Bevalac
accelerator at LBL Berkeley offered projectiles up to 38 Ar, at just the required
energies, 100 MeV ≤ E Lab /nucleon ≤ 2 GeV. The field of “relativistic heavy
ion physics” was born. The topic was confronted, at first, with experimental
methods available both from nuclear and particle physics. It was shown that particle
production (here still restricted to pions and kaons) could indeed be linked to
the equation of state [32] and that, even more spectacularly, the entire “fireball”
of mutually stopped hadrons developed decay modes very closely resembling the
initial predictions of hydrodynamical shock flow modes [33] which directly link
primordial pressure gradients with collective velocity fields of matter streaming out,
again governed by the nuclear/hadronic matter EOS. Actually, both these statements
do, in fact, apply (mutatis mutandis) up to the present ultra-relativistic energies (see
Sects. 7.2–7.4). However it turned out soon that the equation of state at low or even
zero temperature (as required in supernova and neutron star studies) could only be
obtained in a semi-empirical manner [34]. The reason: compression can, in such
collisions, be only accomplished along with temperature and entropy increase. In
an ideal baryon gas exp(s/A) ∝ T 3/2 //, i.e. T 3/2 will grow faster than in a
non-isentropic compression. Thus the reaction dynamics will be sensitive to various
isothermes of the ground state EOS P = f ((, T = 0), staying at T 0,
throughout, and, moreover, not at constant T . Thus a relativistic dynamical mean
field model is required in order to interactively deduce the T = 0 EOS from data
[34]. The EOS result thus remains model dependent.
The ideas concerning creation of a quark gluon plasma arose almost concurrent
with the heavy ion shock compression proposal. In 1974 T.D. Lee formulated the
idea that the non-perturbative vacuum condensates could be “melted down . . . by
distributing high energy or high nucleon density over a relatively large volume”
[35]. Collins and Perry [36] realized that the asymptotic freedom property of QCD
implies the existence of an ultra-hot form of matter with deconfined quarks and
gluons, an idea that gained wide recognition when S. Weinberg [37] proposed an
asymptotic freedom phase at the beginning of “The first Three minutes”. In fact,
this idea of deconfinement by asymptotic freedom (with implied temperature of
several GeV) was correct, but somewhat besides the point, as everybody expected,
likewise, that deconfinement sets in right above the limiting hadron temperature of
R. Hagedorn [38], T H ≈ 160 MeV. A medium existing down to that temperature
would, however, feature an average momentum square transfer Q 2 < 1 GeV 2 , i.e.
be far into the non perturbative domain, and very far from asymptotic freedom.
Right above the hadron to parton transition the “quark gluon plasma” (as it was
named by E. Shuryak [39]) is not a weakly coupled ideal pQCD gas as soon became
R. Stock
resonance—a collective density oscillation also called “breathing mode”—covered
only the parabolic minimum occurring in the related function of energy vs. density
at T = 0, = 0 .
The situation changed around 1970 due to the prediction made by W. Greiner et
al. [31] that nucleus-nucleus collisions, at relatively modest relativistic energies,
would result in shock compression. This mechanism promised to reach matter
densities far beyond those of a mere superposition (i.e. // 0 ≤ 2γ ) of initial
target and projectile densities. Coinciding in time, the newly developed Bevalac
accelerator at LBL Berkeley offered projectiles up to 38 Ar, at just the required
energies, 100 MeV ≤ E Lab /nucleon ≤ 2 GeV. The field of “relativistic heavy
ion physics” was born. The topic was confronted, at first, with experimental
methods available both from nuclear and particle physics. It was shown that particle
production (here still restricted to pions and kaons) could indeed be linked to
the equation of state [32] and that, even more spectacularly, the entire “fireball”
of mutually stopped hadrons developed decay modes very closely resembling the
initial predictions of hydrodynamical shock flow modes [33] which directly link
primordial pressure gradients with collective velocity fields of matter streaming out,
again governed by the nuclear/hadronic matter EOS. Actually, both these statements
do, in fact, apply (mutatis mutandis) up to the present ultra-relativistic energies (see
Sects. 7.2–7.4). However it turned out soon that the equation of state at low or even
zero temperature (as required in supernova and neutron star studies) could only be
obtained in a semi-empirical manner [34]. The reason: compression can, in such
collisions, be only accomplished along with temperature and entropy increase. In
an ideal baryon gas exp(s/A) ∝ T 3/2 //, i.e. T 3/2 will grow faster than in a
non-isentropic compression. Thus the reaction dynamics will be sensitive to various
isothermes of the ground state EOS P = f ((, T = 0), staying at T 0,
throughout, and, moreover, not at constant T . Thus a relativistic dynamical mean
field model is required in order to interactively deduce the T = 0 EOS from data
[34]. The EOS result thus remains model dependent.
The ideas concerning creation of a quark gluon plasma arose almost concurrent
with the heavy ion shock compression proposal. In 1974 T.D. Lee formulated the
idea that the non-perturbative vacuum condensates could be “melted down . . . by
distributing high energy or high nucleon density over a relatively large volume”
[35]. Collins and Perry [36] realized that the asymptotic freedom property of QCD
implies the existence of an ultra-hot form of matter with deconfined quarks and
gluons, an idea that gained wide recognition when S. Weinberg [37] proposed an
asymptotic freedom phase at the beginning of “The first Three minutes”. In fact,
this idea of deconfinement by asymptotic freedom (with implied temperature of
several GeV) was correct, but somewhat besides the point, as everybody expected,
likewise, that deconfinement sets in right above the limiting hadron temperature of
R. Hagedorn [38], T H ≈ 160 MeV. A medium existing down to that temperature
would, however, feature an average momentum square transfer Q 2 < 1 GeV 2 , i.e.
be far into the non perturbative domain, and very far from asymptotic freedom.
Right above the hadron to parton transition the “quark gluon plasma” (as it was
named by E. Shuryak [39]) is not a weakly coupled ideal pQCD gas as soon became