7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
403
Fig. 7.56 The WA98 direct
photon transverse momentum
spectrum for central Pb+Pb
collisions at
√
s = 17.3 GeV.
Also indicated are scaled pA
results above 2.0 GeV/c and
pQCD estimates [243]. From
[224]
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
1
10
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
4.5
WA98
E629
E704
NA3
158 GeV
A
208 Pb +
208 Pb
Central Collisions
p
s
A Results at
1/2
= 19.4 GeV
scaled to s
1/2
= 17.3 GeV
p
s
QCD (
1/2
= 17.3 GeV, = 0),Wong et al.
y
CTEQ4, k
2
T = 0.9 (GeV/c)
2
CTEQ4, No intrinsic k T
p T [GeV/c]
/
1
N v
E
E d
3
N d /
p
3
[c
3 V e
G
2 -
]
subtracting the decay fraction from the total. This was done [224] by WA98 in
central Pb+Pb collisions at top SPS energy; we show their result in Fig. 7.56.
Only upper limits could be obtained at p T ≤ 1.5 GeV/c due to overwhelming
background from π 0 and η decay, thus also obscuring the major spectral domain
in which to look for a direct photon QCD plasma black body radiation source,
i.e. for thermal photons of a fireball at T between T c and about 250 MeV [223].
Several data from p+p and p+A collisions at nearby
√
s and scaled up to central
Pb+Pb are included in Fig. 7.56, at p T ≥ 2 GeV/c, and one sees the Pb+Pb data
in clear excess of such contributions from primordial bremsstrahlung and hard,
pQCD initial partonic collisions [243]. This excess ranges up to about 3.5 GeV/c, in
Fig. 7.56. Above, the hard pQCD initial collision yield dominates [244] over thermal
production.
In contrast to all other primordial high p T pQCD yields (e.g. J//, charm, jet
leading partons) this photon yield is not attenuated in the medium of A+A collisions.
We have shown in Fig. 7.44 the R AA for the PHENIX central Au+Au direct photon
results [181] at RHIC
√
s = 200 GeV, obtained in a background substraction
procedure [245] similar to the one undertaken by WA98. This procedure gives
reliable data at p T > 4.0 GeV/c, at RHIC, and we observe R AA = 1. Hard
initial photons are not attenuated, and there is no sign of any other direct photon
contribution besides the primordial pQCD yield which, in fact, is shown (by R AA =
1) to obey binary scaling. However, there is no hint to plasma thermal radiation
(except for a trend at the very limit of statistical significance, at p T < 4.5 GeV/c) in
this high p T window.
403
Fig. 7.56 The WA98 direct
photon transverse momentum
spectrum for central Pb+Pb
collisions at
√
s = 17.3 GeV.
Also indicated are scaled pA
results above 2.0 GeV/c and
pQCD estimates [243]. From
[224]
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
1
10
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
4.5
WA98
E629
E704
NA3
158 GeV
A
208 Pb +
208 Pb
Central Collisions
p
s
A Results at
1/2
= 19.4 GeV
scaled to s
1/2
= 17.3 GeV
p
s
QCD (
1/2
= 17.3 GeV, = 0),Wong et al.
y
CTEQ4, k
2
T = 0.9 (GeV/c)
2
CTEQ4, No intrinsic k T
p T [GeV/c]
/
1
N v
E
E d
3
N d /
p
3
[c
3 V e
G
2 -
]
subtracting the decay fraction from the total. This was done [224] by WA98 in
central Pb+Pb collisions at top SPS energy; we show their result in Fig. 7.56.
Only upper limits could be obtained at p T ≤ 1.5 GeV/c due to overwhelming
background from π 0 and η decay, thus also obscuring the major spectral domain
in which to look for a direct photon QCD plasma black body radiation source,
i.e. for thermal photons of a fireball at T between T c and about 250 MeV [223].
Several data from p+p and p+A collisions at nearby
√
s and scaled up to central
Pb+Pb are included in Fig. 7.56, at p T ≥ 2 GeV/c, and one sees the Pb+Pb data
in clear excess of such contributions from primordial bremsstrahlung and hard,
pQCD initial partonic collisions [243]. This excess ranges up to about 3.5 GeV/c, in
Fig. 7.56. Above, the hard pQCD initial collision yield dominates [244] over thermal
production.
In contrast to all other primordial high p T pQCD yields (e.g. J//, charm, jet
leading partons) this photon yield is not attenuated in the medium of A+A collisions.
We have shown in Fig. 7.44 the R AA for the PHENIX central Au+Au direct photon
results [181] at RHIC
√
s = 200 GeV, obtained in a background substraction
procedure [245] similar to the one undertaken by WA98. This procedure gives
reliable data at p T > 4.0 GeV/c, at RHIC, and we observe R AA = 1. Hard
initial photons are not attenuated, and there is no sign of any other direct photon
contribution besides the primordial pQCD yield which, in fact, is shown (by R AA =
1) to obey binary scaling. However, there is no hint to plasma thermal radiation
(except for a trend at the very limit of statistical significance, at p T < 4.5 GeV/c) in
this high p T window.