390
R. Stock
Not expected from early estimates of medium opacity based on the picture of a
weakly coupled perturbative QCD medium [172, 202], the interior sections of the
central Au+Au interaction volume must be almost “black” toward high p T parton
transport [194, 195] at
√
s = 200 GeV, also including charm quark propagation
(Fig. 7.43). The remaining signal should thus stem, primarily, from the dilute surface
sections, and from the finite fraction of partons traversing the interior with small, or
zero radiative energy loss, as a consequence of the finite width of the E probability
distribution [193, 194]. Seen in this light, the smooth decrease of R AA with centrality
(Fig. 7.48) should reflect the combined effects, of a decreasing surface to volume
ratio, an increasing effective ˆ
q (due to interior density increase) that confronts the
increasing average geometrical potential path length L (essentially enhancing its
effect), and a thus diminishing fraction of primordial high p T partons experiencing
a small
Not surprisingly, the ideal non abelian QCD relationship of E proportional
to in-medium high p T parton path length L 2 can, thus, not be established from
inclusive high p T quenching data alone. We shall show in the next section that di-jet
primordial production can offer a mechanism of higher resolution. The inclusive
R AA attenuation data, obtained at RHIC, are seen to establish an unexpected,
high opacity of the primordial interaction volume, extending to high p T parton
propagation. The required, high transport coefficient ˆ
q = 14 GeV 2 /fm from
Fig. 7.48, confirms and extends the picture derived from elliptic flow data [167]:
at top RHIC energy the plasma is non-perturbatively, strongly coupled, a new
challenge to lattice QCD [61]. The QGP may be largely composed of colored, stringlike partonic aggregates [203].
7.5.3 Di-jet Production and Attenuation in A+A Collisions
In order to analyze leading parton attenuation in a more constrained situation [204],
one investigates parton tracer attenuation under further geometrical constraints
concerning the in-medium path length L, by means of di-jet analysis, and/or by
varying the primordial parton density that enters ˆ
q via Eq. (7.60) in studies at
different
√
s while maintaining the observational geometrical constraints.
We shall concentrate here on di-jet attenuation data obtained in Au+Au collisions
at top RHIC energy,
√
s = 200 GeV. At this relatively modest energy the initial
pQCD production cross section of leading partons (as described in Eq. (7.51))
reaches up to p T = 25 GeV/c. The ensuing DGLAP shower multiplication initiates
“parton fragmentation” to hadrons [83, 85, 121], each carrying a momentum fraction
z T = p T /p T (primord. parton). The created ensemble of hadrons h belonging to the
observed hadronic jet can be wrapped up by the total fragmentation function
F h (z,
√
s) =
i
dz
z
C i (z,
√
s) D part→h (z,
√
s)
(7.65)
R. Stock
Not expected from early estimates of medium opacity based on the picture of a
weakly coupled perturbative QCD medium [172, 202], the interior sections of the
central Au+Au interaction volume must be almost “black” toward high p T parton
transport [194, 195] at
√
s = 200 GeV, also including charm quark propagation
(Fig. 7.43). The remaining signal should thus stem, primarily, from the dilute surface
sections, and from the finite fraction of partons traversing the interior with small, or
zero radiative energy loss, as a consequence of the finite width of the E probability
distribution [193, 194]. Seen in this light, the smooth decrease of R AA with centrality
(Fig. 7.48) should reflect the combined effects, of a decreasing surface to volume
ratio, an increasing effective ˆ
q (due to interior density increase) that confronts the
increasing average geometrical potential path length L (essentially enhancing its
effect), and a thus diminishing fraction of primordial high p T partons experiencing
a small
Not surprisingly, the ideal non abelian QCD relationship of E proportional
to in-medium high p T parton path length L 2 can, thus, not be established from
inclusive high p T quenching data alone. We shall show in the next section that di-jet
primordial production can offer a mechanism of higher resolution. The inclusive
R AA attenuation data, obtained at RHIC, are seen to establish an unexpected,
high opacity of the primordial interaction volume, extending to high p T parton
propagation. The required, high transport coefficient ˆ
q = 14 GeV 2 /fm from
Fig. 7.48, confirms and extends the picture derived from elliptic flow data [167]:
at top RHIC energy the plasma is non-perturbatively, strongly coupled, a new
challenge to lattice QCD [61]. The QGP may be largely composed of colored, stringlike partonic aggregates [203].
7.5.3 Di-jet Production and Attenuation in A+A Collisions
In order to analyze leading parton attenuation in a more constrained situation [204],
one investigates parton tracer attenuation under further geometrical constraints
concerning the in-medium path length L, by means of di-jet analysis, and/or by
varying the primordial parton density that enters ˆ
q via Eq. (7.60) in studies at
different
√
s while maintaining the observational geometrical constraints.
We shall concentrate here on di-jet attenuation data obtained in Au+Au collisions
at top RHIC energy,
√
s = 200 GeV. At this relatively modest energy the initial
pQCD production cross section of leading partons (as described in Eq. (7.51))
reaches up to p T = 25 GeV/c. The ensuing DGLAP shower multiplication initiates
“parton fragmentation” to hadrons [83, 85, 121], each carrying a momentum fraction
z T = p T /p T (primord. parton). The created ensemble of hadrons h belonging to the
observed hadronic jet can be wrapped up by the total fragmentation function
F h (z,
√
s) =
i
dz
z
C i (z,
√
s) D part→h (z,
√
s)
(7.65)