382
R. Stock
the denominator of R AA , Eq. (7.55). Even in a central trigger configuration this
suggests a lower bound, R AA ≈ 0.15. Nuclear attenuation of high p T pions thus
appears to be almost “maximal”, implying a situation in which the interior sections
of the high energy density fireball feature a very high opacity, i.e. they are almost
“black” to the high p T partons contained in pions.
We remark here, briefly, on a confusing feature that occurs in Fig. 7.42 (and in
the majority of other R AA vs. p T plots emerging from the RHIC experiments):
the pQCD number of collision scaling employed in R AA does not describe pion
production at low p T as we have demonstrated in Sect. 7.2. The entries at p T ≤
3 GeV/c are thus besides the point, and so are pQCD guided model predictions, as
shown here [176].
Nuclear modification analysis at RHIC covers, by now, the high p T production of
a multitude of mesonic and baryonic species [177], most remarkably even including
the charmed D meson which is measured via electrons from semi-leptonic heavy
flavor decay by PHENIX [175] and STAR [178]. Figure 7.43 illustrates the first
PHENIX results from central Au+Au at
√
s = 200 GeV, R AA falling to about
0.3 at 5 GeV/c. Heavy flavor attenuation thus resembles that of light quarks, as is
also attested by the predictions of in-medium parton transport models [179, 180]
included in Fig. 7.43, which cast the medium opacity into an effective parton
transport coefficient ˆ
q which is seen here to approach a value 14 GeV 2 /c at high p T ,
again corresponding to a highly opaque medium. We shall describe this approach in
more detail below.
A most fundamental cross-check of the in-medium attenuation picture of color
charged partons consists in measuring R AA for primordial, “direct” photons.
Fig. 7.43 The nuclear
modification factor R AA vs.
p T for electrons from
semi-leptonic decays of
heavy flavor (mostly D)
mesons in central Au+Au
collisions at
√
s = 200 GeV
[175]; with calculations of in
medium energy loss using
different attenuation models
[179, 180]
R. Stock
the denominator of R AA , Eq. (7.55). Even in a central trigger configuration this
suggests a lower bound, R AA ≈ 0.15. Nuclear attenuation of high p T pions thus
appears to be almost “maximal”, implying a situation in which the interior sections
of the high energy density fireball feature a very high opacity, i.e. they are almost
“black” to the high p T partons contained in pions.
We remark here, briefly, on a confusing feature that occurs in Fig. 7.42 (and in
the majority of other R AA vs. p T plots emerging from the RHIC experiments):
the pQCD number of collision scaling employed in R AA does not describe pion
production at low p T as we have demonstrated in Sect. 7.2. The entries at p T ≤
3 GeV/c are thus besides the point, and so are pQCD guided model predictions, as
shown here [176].
Nuclear modification analysis at RHIC covers, by now, the high p T production of
a multitude of mesonic and baryonic species [177], most remarkably even including
the charmed D meson which is measured via electrons from semi-leptonic heavy
flavor decay by PHENIX [175] and STAR [178]. Figure 7.43 illustrates the first
PHENIX results from central Au+Au at
√
s = 200 GeV, R AA falling to about
0.3 at 5 GeV/c. Heavy flavor attenuation thus resembles that of light quarks, as is
also attested by the predictions of in-medium parton transport models [179, 180]
included in Fig. 7.43, which cast the medium opacity into an effective parton
transport coefficient ˆ
q which is seen here to approach a value 14 GeV 2 /c at high p T ,
again corresponding to a highly opaque medium. We shall describe this approach in
more detail below.
A most fundamental cross-check of the in-medium attenuation picture of color
charged partons consists in measuring R AA for primordial, “direct” photons.
Fig. 7.43 The nuclear
modification factor R AA vs.
p T for electrons from
semi-leptonic decays of
heavy flavor (mostly D)
mesons in central Au+Au
collisions at
√
s = 200 GeV
[175]; with calculations of in
medium energy loss using
different attenuation models
[179, 180]