7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
339
Fig. 7.16 Saturation model fit [63] applied to RHIC charged hadron multiplicity data at midrapidity normalized by number of participant pairs, at various energies [82]. Also shown is an
extrapolation to pp data and a prediction for minimum bias Pb+Pb collisions at LHC energy,
√
s = 5500 GeV
√
s = 19.6, 130 and 200 GeV [82], also including a prediction for LHC energy. Note
that the factorization of energy and centrality dependence, implied by the RHIC data
[52], is well captured by Eq. (7.11) and the resulting fits in Fig. 7.16. Furthermore,
the steeper slope, predicted for N part ≤ 60 (not covered by the employed data set),
interpolates to the corresponding pp and pp data, at N part = 2. It resembles the
pattern observed in the NA49 data (Fig. 7.13) for small N part collisions of light
A+A systems, at
√
s = 17–20 GeV, and may be seen, to reflect the onset of QCD
saturation. Finally we note that the conclusions of the above, partially heuristic
approach [63], represented by Eqs. (7.13)–(7.16), have been backed up by the CGC
theory of McLerran and Venugopulan [64, 65, 75], predictions of which we have
illustrated in Fig. 7.7.
Bulk hadron production in AA collisions at high
√
s can be related, via the
assumption of universality of high energy QCD scattering, to the phenomenon of
geometric scaling first observed in HERA deep inelastic ep cross sections. The
underlying feature is a QCD saturation effect arising from the diverging areal
parton density, as confronted with the limited areal resolution Q 2 , inherent in
the considered scattering process. The “saturation scale” Q 2
s (x, A) captures the
condition that a single partonic charge source within the transverse partonic density
profile can just be resolved by a sufficiently high Q 2 . Bulk hadron production in
A+A collisions falls below this scale.
7.2.5 Transverse Phase Space: Equilibrium and the QGP State
At RHIC energy,
√
s = 200 GeV, the Au+Au collision reactants are longitudinally
contracted discs. At a nuclear radius R ≈ A 1/3 fm and Lorentz γ ≈ 100 their
primordial interpenetration phase ends at time τ 0 ≤ 0.15 fm/c. This time scale is
339
Fig. 7.16 Saturation model fit [63] applied to RHIC charged hadron multiplicity data at midrapidity normalized by number of participant pairs, at various energies [82]. Also shown is an
extrapolation to pp data and a prediction for minimum bias Pb+Pb collisions at LHC energy,
√
s = 5500 GeV
√
s = 19.6, 130 and 200 GeV [82], also including a prediction for LHC energy. Note
that the factorization of energy and centrality dependence, implied by the RHIC data
[52], is well captured by Eq. (7.11) and the resulting fits in Fig. 7.16. Furthermore,
the steeper slope, predicted for N part ≤ 60 (not covered by the employed data set),
interpolates to the corresponding pp and pp data, at N part = 2. It resembles the
pattern observed in the NA49 data (Fig. 7.13) for small N part collisions of light
A+A systems, at
√
s = 17–20 GeV, and may be seen, to reflect the onset of QCD
saturation. Finally we note that the conclusions of the above, partially heuristic
approach [63], represented by Eqs. (7.13)–(7.16), have been backed up by the CGC
theory of McLerran and Venugopulan [64, 65, 75], predictions of which we have
illustrated in Fig. 7.7.
Bulk hadron production in AA collisions at high
√
s can be related, via the
assumption of universality of high energy QCD scattering, to the phenomenon of
geometric scaling first observed in HERA deep inelastic ep cross sections. The
underlying feature is a QCD saturation effect arising from the diverging areal
parton density, as confronted with the limited areal resolution Q 2 , inherent in
the considered scattering process. The “saturation scale” Q 2
s (x, A) captures the
condition that a single partonic charge source within the transverse partonic density
profile can just be resolved by a sufficiently high Q 2 . Bulk hadron production in
A+A collisions falls below this scale.
7.2.5 Transverse Phase Space: Equilibrium and the QGP State
At RHIC energy,
√
s = 200 GeV, the Au+Au collision reactants are longitudinally
contracted discs. At a nuclear radius R ≈ A 1/3 fm and Lorentz γ ≈ 100 their
primordial interpenetration phase ends at time τ 0 ≤ 0.15 fm/c. This time scale is