7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
397
energy” incident proton does not preserve its identity, breaking up into relatively
soft partons in the course of traversing the target nucleus. Their coupling to the
surrounding cold baryonic target matter is weak, and possible collective transverse
compressional waves are swiftly dissipated by the cold, highly viscous [61] hadronic
medium. The shock compression and Mach-cone emission mechanism was then
revived by Greiner et al. [217] for central 20 Ne + U collisions at Bevalac energy
[218]. The recent RHIC observations in Fig. 7.53 have been called “conical flow”
[219]; they demonstrate, again, the strong coupling to the medium, of even a
20 GeV/c leading parton [220].
7.6 Vector Meson and Direct Photon Production:
Penetrating Probes
This chapter is devoted to three observables that have been profoundly studied
already at the SPS, as tracers of the created fireball medium:
1. J// “charmonium” production, as suppressed in a high T QGP medium,
2. “direct” photons as black body T sensors of the QGP state;
3. ρ meson in medium production, studied by di-lepton decay.
These three observables represent an internally connected set of ideas of high
density QCD matter diagnostics: all serve as medium tracers but in a complementary
way. Matsui and Satz [41] realized that at the modest top SPS energy, 17.3 <
√
s < 20 GeV, the production rate of cc pairs (that would in part develop toward
charmonium production, J//, , , etc.) was so low that only the primordial, first
generation nucleon-nucleon collisions in an A+A event would create a measurable
yield (nothing after thermalization). I.e. the primordial cc yield would be wellestimated by A 4/3 σ pp (cc). Initially produced in a superposition of states including
color triplets and octets [221] the emerging cc pairs thus co-travel with the
developing high energy density fireball medium, as tracers, on their way toward
J// or and D, ¯
D formation. Attenuation, characteristic of medium properties,
will break up leading cc pairs resulting in a suppression of the eventually observed
J// yield: another “quenching” observable.
As the suppression of J/ψ is related to medium temperature and density, the
extent of charmonium quenching should also be related to the rate of black body
thermal fireball radiation, by photon emission via elementary qq → gγ and
qg → qγ processes in the plasma [222, 223]. Photons leave the interaction volume
essentially un-rescattered, and their radiative intensity, proportional to T 4 , makes
them an ideal probe of the initial fireball temperature. This thus could be an ideal
diagnostics of the early deconfined matter, but it is difficult to disentangle from a
multitude of concurrent low p T photon sources [224], most prominently π 0 → 2γ
decay, with cross sections higher by several orders of magnitude. The thermal
397
energy” incident proton does not preserve its identity, breaking up into relatively
soft partons in the course of traversing the target nucleus. Their coupling to the
surrounding cold baryonic target matter is weak, and possible collective transverse
compressional waves are swiftly dissipated by the cold, highly viscous [61] hadronic
medium. The shock compression and Mach-cone emission mechanism was then
revived by Greiner et al. [217] for central 20 Ne + U collisions at Bevalac energy
[218]. The recent RHIC observations in Fig. 7.53 have been called “conical flow”
[219]; they demonstrate, again, the strong coupling to the medium, of even a
20 GeV/c leading parton [220].
7.6 Vector Meson and Direct Photon Production:
Penetrating Probes
This chapter is devoted to three observables that have been profoundly studied
already at the SPS, as tracers of the created fireball medium:
1. J// “charmonium” production, as suppressed in a high T QGP medium,
2. “direct” photons as black body T sensors of the QGP state;
3. ρ meson in medium production, studied by di-lepton decay.
These three observables represent an internally connected set of ideas of high
density QCD matter diagnostics: all serve as medium tracers but in a complementary
way. Matsui and Satz [41] realized that at the modest top SPS energy, 17.3 <
√
s < 20 GeV, the production rate of cc pairs (that would in part develop toward
charmonium production, J//, , , etc.) was so low that only the primordial, first
generation nucleon-nucleon collisions in an A+A event would create a measurable
yield (nothing after thermalization). I.e. the primordial cc yield would be wellestimated by A 4/3 σ pp (cc). Initially produced in a superposition of states including
color triplets and octets [221] the emerging cc pairs thus co-travel with the
developing high energy density fireball medium, as tracers, on their way toward
J// or and D, ¯
D formation. Attenuation, characteristic of medium properties,
will break up leading cc pairs resulting in a suppression of the eventually observed
J// yield: another “quenching” observable.
As the suppression of J/ψ is related to medium temperature and density, the
extent of charmonium quenching should also be related to the rate of black body
thermal fireball radiation, by photon emission via elementary qq → gγ and
qg → qγ processes in the plasma [222, 223]. Photons leave the interaction volume
essentially un-rescattered, and their radiative intensity, proportional to T 4 , makes
them an ideal probe of the initial fireball temperature. This thus could be an ideal
diagnostics of the early deconfined matter, but it is difficult to disentangle from a
multitude of concurrent low p T photon sources [224], most prominently π 0 → 2γ
decay, with cross sections higher by several orders of magnitude. The thermal